重点：
MySQL 的 三种安装方式：包安装，二进制安装，源码编译安装。
MySQL 的 基本使用
MySQL 多实例
DDLcreate alter drop
DML insert update delete
DQL select
3.5）DDL 语句
表：二维关系
设计表：遵循规范
定义：字段，索引
字段：字段名，字段数据类型，修饰符
约束，索引：应该创建在经常用作查询条件的字段上
3.5.1） 创建表
参考文档： MySQL :: MySQL 8.0 Reference Manual :: 13.1.20 CREATE TABLE Statement
// 创建表
CREATE TABLE
// 获取帮助
HELP CREATE TABLE
外键管理参考文档：MySQL :: MySQL 8.0 Reference Manual :: 13.1.20.5 FOREIGN KEY Constraints
创建表的方法
（1）直接创建
CREATE TABLE [IF NOT EXISTS] 'tbl_name' (col1 type1 修饰符, col2 type2 修饰符, ...)
// 字段信息
col type1
PRIMARY KEY(col1,...)
INDEX(col1, ...)
UNIQUE KEY(col1, ...)
// 表选项：
ENGINE [=] engine_name
ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT}
注意：
Storage Engine 是指表类型，也即在表创建时指明其使用的存储引擎
同一库中不同表可以使用不同的存储引擎
同一个库中表建议要使用同一种存储引擎类型
范例： 创建表
创建一个名为 student 的数据表
定义一个名为 id 的字段
SMALLINT整数数据类型 最大值 65535
UNSIGNED仅存储正 整数
PRIMARY KEY表的主 键，每个 主键值都 必须是唯一且不为空。
AUTO_INCREMENT值会 自动递增
定义一个名为 name 的字段
VARCHAR(10)字符串类型 存储的 最大字符数为 1 0。
NOT NULL 不允许存储空值
定义一个名为 age 的字段
TINYINT整数数据类型最 大值 255
UNSIGNED仅存储正 整数
创建一个名为 gender 的字段
ENUM('M','F')数据类型 为 ENUM 枚举类型
DEFAULT 'M'默认值将 被设置 为 'M'
创建表的 SQL 语句的结尾部分
ENGINE=InnoDB指定存 储引擎为 I nnoDB
UTO_INCREMENT=10指定 自增列的初始值为 10。这表示在向具有自增功能的列中插入新行时，首个自增值将为 10，接 下来的自增值将逐次增加 1。
EFAULT CHARSET=utf8指 定表的默认字符集为 utf8。（ 不默认使用数据库的默认字符集 ）
CREATE TABLE student (
id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(10) NOT NULL,
age TINYINT UNSIGNED,
gender ENUM('M','F') DEFAULT 'M'
)ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
// 创建数据库
MariaDB [(none)]> create database db1;
Query OK, 1 row affected (0.00 sec)
// 进入数据库
MariaDB [(none)]> use db1;
Database changed
// 创建数据表
MariaDB [db1]> CREATE TABLE student (
-> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
-> name VARCHAR(10) NOT NULL,
-> age TINYINT UNSIGNED,
-> gender ENUM('M','F') DEFAULT 'M'
-> )ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.00 sec)
// 验证
MariaDB [db1]> DESC student;
+--------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(10) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
| gender | enum('M','F') | YES | | M | |
+--------+----------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
'第一个注意点' // 字段定义的重要性
// 插入数据 ( 注: 插入的值超过了 10 位, 有 12 位 '会被截断插入' )
MariaDB [db1]> insert student(name,age) values('wangxiaochun',20);
Query OK, 1 row affected, 1 warning (0.01 sec) ( 报错有一个 warning )
// 查看 warning 信息
MariaDB [db1]> show warnings;
+---------+------+-------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------+
| Warning | 1265 | Data truncated for column 'name' at row 1 |
+---------+------+-------------------------------------------+
1 row in set (0.00 sec)
// 查询表
MariaDB [db1]> select * from student;
+----+------------+------+--------+
| id | name | age | gender |
+----+------------+------+--------+
| 10 | wangxiaoch | 20 | M | # 多的字符没有被插入完全
+----+------------+------+--------+
1 row in set (0.00 sec)
// 插入数据
MariaDB [db1]> insert student(name,age,gender) values('xiaohong',18,'f');
Query OK, 1 row affected (0.001 sec)
// 查询表
MariaDB [db1]> select * from student;
+----+------------+------+--------+
| id | name | age | gender |
+----+------------+------+--------+
| 10 | wangxiaoch | 20 | M |
| 11 | xiaohong | 18 | F |
+----+------------+------+--------+
2 rows in set (0.001 sec)
'第二个注意点' // 数据库字符集的重要性
// 插入数据 ( 插入汉字 )
// 如果数据库字符集为 latin1, 会报错 ( 数值会乱码 )
MariaDB [db1]> insert student(name,age) values('王俊',18);
Query OK, 1 row affected, 1 warning (0.01 sec)
// 查询表
MariaDB [db1]> select * from student;
+----+------------+------+--------+
| id | name | age | gender |
+----+------------+------+--------+
| 10 | wangxiaoch | 20 | M |
| 11 | xiaohong | 18 | F |
| 12 | ?? | 18 | M | // 如果数据库字符集为 latin1 ( 数值会乱码 )
+----+------------+------+--------+
3 rows in set (0.001 sec)
//
CREATE TABLE employee (id int UNSIGNED NOT NULL ,name VARCHAR(20) NOT NULL,age tinyint UNSIGNED,PRIMARY KEY(id,name));
// 查询 'student' 表的信息
// 这样的查询通常用于查看表的基本信息以及一些统计数据
// 以便于数据库管理员了解表的使用情况和性能特征
SHOW TABLE STATUS LIKE 'student';
SHOW TABLE STATUS LIKE 'student'\G
范例： auto_increment 属性
MariaDB [hellodb]> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.001 sec)
MariaDB [hellodb]> SET @@auto_increment_increment=10;
Query OK, 0 rows affected (0.001 sec)
MariaDB [hellodb]> SET @@auto_increment_offset=3;
Query OK, 0 rows affected (0.000 sec)
MariaDB [hellodb]> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 3 |
+--------------------------+-------+
2 rows in set (0.001 sec)
MariaDB [hellodb]> CREATE TABLE autoinc1 (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.004 sec)
MariaDB [hellodb]> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.001 sec)
Records: 4 Duplicates: 0 Warnings: 0
MariaDB [hellodb]> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 3 |
| 13 |
| 23 |
| 33 |
+-----+
4 rows in set (0.000 sec)
范例：
MariaDB [db1]> create table testdate (id int auto_increment primary key,date timestamp DEFAULT CURRENT_TIMESTAMP NOT NULL);
MariaDB [db1]> insert testdate()values()()();
MariaDB [db1]> select * from testdate;
+----+---------------------+
| id | date |
+----+---------------------+
| 1 | 2020-09-23 08:41:43 |
| 2 | 2020-09-23 08:41:43 |
| 3 | 2020-09-23 08:41:43 |
+----+---------------------+
3 row in set (0.000 sec)
（2）通过查询现存表创建；新表会被直接插入查询而来的数据
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name [(create_definition,...)] [table_options] [partition_options] select_statement
范例：
MariaDB [db1]> create table user select user,host,password from mysql.user;
Query OK, 4 rows affected (0.008 sec)
Records: 4 Duplicates: 0 Warnings: 0
MariaDB [db1]> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| student |
| user |
+---------------+
2 rows in set (0.000 sec)
MariaDB [db1]> desc user;
+----------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------+------+-----+---------+-------+
| user | char(80) | NO | | | |
| host | char(60) | NO | | | |
| password | char(41) | NO | | | |
+----------+----------+------+-----+---------+-------+
（3）通过复制现存的表的表结构创建，但不复制数据
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name { LIKE old_tbl_name | (LIKE old_tbl_name) }
范例：
// 查看表结构
MariaDB [db1]> desc student;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
| gender | enum('M','F') | YES | | M | |
+--------+---------------------+------+-----+---------+----------------+
4 rows in set (0.001 sec)
MariaDB [db1]> create table teacher like student;
Query OK, 0 rows affected (0.006 sec)
MariaDB [db1]> desc teacher;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
| gender | enum('M','F') | YES | | M | |
+--------+---------------------+------+-----+---------+----------------+
4 rows in set (0.001 sec)
范例： 创建外键表
mysql> create table school ( id int primary key auto_increment,name varchar(10));
mysql> create table teacher(id int primary key auto_increment,name varchar(10), school_id int,foreign key(school_id) references school(id));
mysql> desc school;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int | NO | PRI | NULL | auto_increment |
| name | varchar(10) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> desc teacher;
+-----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+----------------+
| id | int | NO | PRI | NULL | auto_increment |
| name | varchar(10) | YES | | NULL | |
| school_id | int | YES | MUL | NULL | |
+-----------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> show create table teacher;
mysql> insert school values(0,'magedu'),(0,'wangedu');
mysql> select * from school;
+----+---------+
| id | name |
+----+---------+
| 1 | magedu |
| 2 | wangedu |
+----+---------+
2 rows in set (0.00 sec)
mysql> insert teacher values(0,'xiaoming',1);
mysql> insert teacher values(0,'xiaohong',2);
Query OK, 1 row affected (0.00 sec)
mysql> select * from teacher;
+----+----------+-----------+
| id | name | school_id |
+----+----------+-----------+
| 1 | xiaoming | 1 |
| 2 | xiaohong | 2 |
+----+----------+-----------+
2 rows in set (0.00 sec)
mysql> insert teacher values(0,'xiaobai',3);
3.5.2） 表查看
查看表：
SHOW TABLES [FROM db_name]
查看表创建命令：
SHOW CREATE TABLE tbl_name
查看表结构：
DESC [db_name.]tb_name
SHOW COLUMNS FROM [db_name.]tb_name
查看表状态：
SHOW TABLE STATUS LIKE 'tbl_name'
查看支持的 engine 类型
SHOW ENGINES;
范例：
MariaDB [db1]> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| student |
+---------------+
1 row in set (0.000 sec)
范例：
// 查看表结构 方法 1
MariaDB [db1]> desc student;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
| gender | enum('M','F') | YES | | M | |
+--------+---------------------+------+-----+---------+----------------+
4 rows in set (0.001 sec)
// 查看表结构 方法 2
MariaDB [db1]> SHOW COLUMNS FROM student;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
| gender | enum('M','F') | YES | | M | |
+--------+---------------------+------+-----+---------+----------------+
4 rows in set (0.001 sec)
范例：
MariaDB [db1]> show create table student;
范例：
// 查询 'student' 表的信息
MariaDB [db1]> SHOW TABLE STATUS LIKE 'student'\G
*************************** 1. row ***************************
Name: student
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: 10
Create_time: 2020-02-17 11:35:29
Update_time: NULL
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
Max_index_length: 0
Temporary: N
1 row in set (0.001 sec)
查看库中所有表状态
SHOW TABLE STATUS FROM db_name
范例：
// 查询 'student' 表的信息
MariaDB [db1]> SHOW TABLE STATUS FROM db1\G
*************************** 1. row ***************************
Name: employee
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: 2020-02-17 11:43:21
Update_time: NULL
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
Max_index_length: 0
Temporary: N
*************************** 2. row ***************************
Name: student
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: 10
Create_time: 2020-02-17 11:35:29
Update_time: NULL
Check_time: NULL
Collation: latin1_swedish_ci
Checksum: NULL
Create_options:
Comment:
Max_index_length: 0
Temporary: N
2 rows in set (0.001 sec)
3.5.3） 修改和删除表
ALTER TABLE 'tbl_name'
// 字段:
// 添加字段: add
ADD col1 data_type [FIRST|AFTER col_name]
// 删除字段: drop
// 修改字段:
alter(默认值), change(字段名), modify(字段属性)
查看修改表帮助
Help ALTER TABLE
删除表
DROP TABLE [IF EXISTS] 'tbl_name';
修改表范例
修改表名
ALTER TABLE students RENAME s1;
添加字段
// ALTER TABLE s1: 指定要修改的表名为 s1
// ADD: 表明要添加一个新的列
// phone: 这是新列的名称
// char(11): 指定新列的数据类型为字符型, 长度为 11.
// NOT NULL: 这是一个约束, 表示这个字段不能为 NULL, 即这个字段必须有值.
ALTER TABLE s1 ADD phone char(11) NOT NULL;
修改字段类型
// ALTER TABLE s1: 指定要修改的表名为 s1
// MODIFY: 表明要修改现有列的数据类型
// phone: 这是要修改的列的名称
// int: 这是要将列修改为的新数据类型
ALTER TABLE s1 MODIFY phone int;
修改字段名称和类型
// ALTER TABLE s1: 指定要修改的表名为 s1
// CHANGE COLUMN: 表明要修改现有的列
// phone: 这是要修改的列的当前名称
// mobile: 这是要将列修改为的新名称
// varchar(11): 这是要将列修改为的新数据类型, 即字符型, 长度为 11
ALTER TABLE s1 CHANGE COLUMN phone mobile varchar(11);
删除字段
ALTER TABLE students DROP age;
查看表结构
DESC students;
# 新建表无主键, 添加和删除主键
CREATE TABLE t1 SELECT * FROM students;
ALTER TABLE t1 add primary key (stuid);
ALTER TABLE t1 drop primary key;
# 添加外键
ALTER TABLE students add foreign key(TeacherID) references teachers(tid);
# 删除外键
SHOW CREATE TABLE students # 查看外键名
ALTER TABLE students drop foreign key <外键名>;
3.6）DML 语句
DML：INSERT，DELETE，UPDATE
3.6.1） INSERT 语句
功能： 一次插入一行或多行数据
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name [(col_name,...)]
{VALUES | VALUE} ({expr | DEFAULT},...),(...),...
[ ON DUPLICATE KEY UPDATE #如果重复更新之
col_name=expr
[, col_name=expr] ... ]
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
SET col_name={expr | DEFAULT}, ...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name [(col_name,...)]
SELECT ...
[ ON DUPLICATE KEY UPDATE
col_name=expr
[, col_name=expr] ... ]
简化写法：
INSERT tbl_name [(col1,...)] VALUES (val1,...), (val21,...)
范例： 全值 插入
字段赋值需完整排序
// id 字段定义 0 没有关系 ( 该字段会自动递增 )
// gender 字段定义 default 没有关系 ( 该字段会默认定义为 M )
mysql> insert student values(0,'wang',18,default);
Query OK, 1 row affected (0.01 sec)
mysql> select * from student;
+----+------+------+--------+
| id | name | age | gender |
+----+------+------+--------+
| 1 | wang | 18 | M |
+----+------+------+--------+
1 row in set (0.00 sec)
范例： 部分列 插入
字段名与字段赋值一一对应
mysql> insert student(name,age) values('zhang',20);
Query OK, 1 row affected (0.01 sec)
// id 字段定义 default 没有关系 ( 该字段会自动递增 )
mysql> insert student(id,name,age) values(default,'li',19);
Query OK, 1 row affected (0.00 sec)
// id 字段定义 null 也没有关系 ( 该字段会自动递增 )
mysql> insert student(id,name,gender) values(null,'zhao','F');
Query OK, 1 row affected (0.00 sec)
mysql> select * from student;
+----+-------+------+--------+
| id | name | age | gender |
+----+-------+------+--------+
| 1 | wang | 18 | M |
| 2 | zhang | 20 | M |
| 3 | li | 19 | M |
| 4 | zhao | NULL | F |
+----+-------+------+--------+
4 rows in set (0.00 sec)
3.6.2） UPDATE 语句
语法：
UPDATE [LOW_PRIORITY] [IGNORE] table_reference
SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
注意： 一定要有限制条件，否则将修改指定字段的所有行
// 注意: 一定要有限制条件, 否则将修改指定字段的所有行
update student set gender='F';
//
'整数型'类型: 无需使用引号包含
'字符串'类型: 需要使用引号包含
update student set gender='F' where id=5;
update student set gender='F' where name='小红';
可利用 MySQL 选项避免此错误：
mysql -U | --safe-updates | --i-am-a-dummy
[root@centos8 ~] vim /etc/my.cnf
[mysql]
safe-updates
4.6.3） DELETE 语句
删除表中数据，但不会自动缩减数据文件的大小。
DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
可先排序再指定删除的行数
注意： 一定要有限制条件，否则将清空表中的所有数据。
// 注意: 一定要有限制条件, 否则将清空表中的所有数据.
delete from student where id=5;
如果想清空表，保留表结构，也可以使用下面语句，此语句会自动缩减数据文件的大小。
TRUNCATE TABLE tbl_name;
缩减表大小
OPTIMIZE TABLE tb_name
范例： 删除数据可以使用逻辑删除，添加一个标识字段实现，删除数据即修改标识字段
mysql> alter table student add is_del bool default false;
mysql> desc student;
+--------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+------------------+------+-----+---------+----------------+
| id | int unsigned | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | tinyint unsigned | YES | | NULL | |
| gender | enum('M','F') | YES | | M | |
| is_del | tinyint(1) | YES | | 0 | |
+--------+------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
3.7）DQL 语句
3.7.1）单表查询
MySQL 基本总结 （ 学习ing ）
MySQL 常用命令：开放平台
MySQL 单表查询：开放平台
MySQL 多表查询：开放平台
MySQL 多表查询：开放平台
status // 查询 MYSQL 版本
show databases; // 查看所有数据库
use xxx; // 使用数据库
show tables; // 查看所有数据表
// 查询 ( 能查询部分字段的, 千万不要查询 *, 这样会增加查询时间 )
select * from 表名; // 查询一个表中的所有数据
select 字段1,字段2,字段3 from 表名; // 查询指定字段的数据
// 给字段取别名
select name from students;
select name as 姓名 from students;
// 给表取别名
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[SQL_CACHE | SQL_NO_CACHE]
select_expr [, select_expr ...]
[FROM table_references
[WHERE where_condition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_condition]
[ORDER BY {col_name | expr | position}
[ASC | DESC], ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[FOR UPDATE | LOCK IN SHARE MODE]
说明：
字段显示可以使用别名：
col1 AS alias1，col2 AS alias2，...
WHERE 子句：指明过滤条件以实现 "选择" 的功能：
过滤条件：布尔型表达式
算术操作符：+, -, *, /, %
比较操作符：=,<=>（相等或都为空）, <>, !=(非标准SQL), >, >=, <, <=
范例查询: BETWEEN min_num AND max_num
不连续的查询: IN (element1, element2, ...)
空查询: IS NULL, IS NOT NULL
DISTINCT 去除重复行，范例：SELECT DISTINCT gender FROM students;
模糊查询: LIKE 使用 % 表示任意长度的任意字符 _ 表示任意单个字符
RLIKE：正则表达式，索引失效，不建议使用
REGEXP：匹配字符串可用正则表达式书写模式，同上
逻辑操作符：NOT，AND，OR，XOR
GROUP BY：根据指定的条件把查询结果进行 "分组" 以用于做 "聚合" 运算
常见聚合函数： count()，sum()，max()，min()，avg()
注意：聚合函数不对 null 统计
HAVING：对分组聚合运算后的结果指定过滤条件
一旦分组 group by ，select 语句后只跟分组的字段，聚合函数
ORDER BY：根据指定的字段对查询结果进行排序
升序：ASC
降序：DESC
LIMIT [[offset,]row_count]：对查询的结果进行输出行数数量限制，跳过 offset，显示 row_count 行，offset 默为值为 0
对查询结果中的数据请求施加 "锁"
FOR UPDATE：写锁，独占或排它锁，只有一个读和写操作
LOCK IN SHARE MODE：读锁，共享锁，同时多个读操作
( '简单查询' )
// 查询 student 表所有字段的数据
select * from student;
// 查询 student 表所有字段的数据
// 然后过滤出那些性别字段值为 'F' 的记录
select * from student where gender='F';
// 查询 student 表 name,age,phone 字段的数据
// 然后过滤出那些性别字段值为 'F' 的记录
select name,age,phone from student where gender='F';
实验环境
MariaDB [(none)]> source /root/hellodb_innodb.sql
MariaDB [hellodb]> show tables;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes |
| coc |
| courses |
| scores |
| students |
| teachers |
| toc |
+-------------------+
7 rows in set (0.00 sec)
MariaDB [hellodb]> show create table students;
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| students | CREATE TABLE `students` (
`StuID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`Name` varchar(50) NOT NULL,
`Age` tinyint(3) unsigned NOT NULL,
`Gender` enum('F','M') NOT NULL,
`ClassID` tinyint(3) unsigned DEFAULT NULL,
`TeacherID` int(10) unsigned DEFAULT NULL,
PRIMARY KEY (`StuID`)
) ENGINE=InnoDB AUTO_INCREMENT=26 DEFAULT CHARSET=utf8 |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
MariaDB [hellodb]> select * from students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)
案例演示
// 查询年龄为 20 以下的行信息
select * from students where age <=20;
// 查询年龄为 20 - 30 之间的行信息
select * from students where age >=20 and age <=30;
select * from students where age between 20 and 30;
// 查询年龄为 18,20,22 的行信息
select * from students where age in (18,20,22);
// 只查看年龄列的行信息
select age from students;
select distinct age from students; # 只查看不同值 ( 去重操作 )
// 查看 classid 为 NULL 值的行信息
select * from students where classid is null;
select * from students where classid is not null; # 不为 NULL 值
// LIKE 模糊匹配
select * from students where name = 'Xi Ren'; # 精确查找
select * from students where name like 'Xi%'; # 模糊查找
select * from students where name like '%Xi%'; # 模糊查找
范例：
MariaDB [hellodb]> select password("hello world") ;
+-------------------------------------------+
| password("hello world") |
+-------------------------------------------+
| *67BECF85308ACF0261750DA1075681EE5C412F05 |
+-------------------------------------------+
1 row in set (0.000 sec)
MariaDB [hellodb]> select md5("hello world") ;
+----------------------------------+
| md5("hello world") |
+----------------------------------+
| 5eb63bbbe01eeed093cb22bb8f5acdc3 |
+----------------------------------+
1 row in set (0.000 sec)
范例：字段别名
MariaDB [hellodb]> select stuid 学员ID,name as 姓名,gender 性别 from students;
+----------+---------------+--------+
| 学员ID | 姓名 | 性别 |
+----------+---------------+--------+
| 1 | Shi Zhongyu | M |
| 2 | Shi Potian | M |
| 3 | Xie Yanke | M |
| 4 | Ding Dian | M |
| 5 | Yu Yutong | M |
| 6 | Shi Qing | M |
| 7 | Xi Ren | F |
| 8 | Lin Daiyu | F |
| 9 | Ren Yingying | F |
| 10 | Yue Lingshan | F |
| 11 | Yuan Chengzhi | M |
| 12 | Wen Qingqing | F |
| 13 | Tian Boguang | M |
| 14 | Lu Wushuang | F |
| 15 | Duan Yu | M |
| 16 | Xu Zhu | M |
| 17 | Lin Chong | M |
| 18 | Hua Rong | M |
| 19 | Xue Baochai | F |
| 20 | Diao Chan | F |
| 21 | Huang Yueying | F |
| 22 | Xiao Qiao | F |
| 23 | Ma Chao | M |
| 24 | Xu Xian | M |
| 25 | Sun Dasheng | M |
+----------+---------------+--------+
25 rows in set (0.000 sec)
范例：简单查询
DESC students;
INSERT INTO students VALUES(1,'tom'，'m'),(2,'alice','f');
INSERT INTO students(id,name) VALUES(3,'jack'),(4,'allen');
SELECT * FROM students WHERE id < 3;
SELECT * FROM students WHERE gender='m';
SELECT * FROM students WHERE gender IS NULL;
SELECT * FROM students WHERE gender IS NOT NULL;
SELECT * FROM students ORDER BY name DESC LIMIT 2;
SELECT * FROM students ORDER BY name DESC LIMIT 1,2;
SELECT * FROM students WHERE id >=2 and id <=4
SELECT * FROM students WHERE BETWEEN 2 AND 4
SELECT * FROM students WHERE name LIKE 't%'
SELECT * FROM students WHERE name RLIKE '.*[lo].*';
SELECT id stuid,name as stuname FROM students
select * from students where classid in (1,3,5);
select * from students where classid not in (1,3,5);
范例：判断是否为 NULL
MariaDB [hellodb]> select * from students where classid is null;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+-------------+-----+--------+---------+-----------+
2 rows in set (0.002 sec)
MariaDB [hellodb]> select * from students where classid <=> null;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+-------------+-----+--------+---------+-----------+
2 rows in set (0.000 sec)
MariaDB [hellodb]> select * from students where classid is not null;
# ifnull 函数判断指定的字段是否为空值,如果空值则使用指定默认值
mysql> select stuid,name,ifnull(classID,'无班级') from students where classid is null;
+-------+-------------+-----------------------------+
| stuid | name | ifnull(classID,'无班级') |
+-------+-------------+-----------------------------+
| 24 | Xu Xian | 无班级 |
| 25 | Sun Dasheng | 无班级 |
+-------+-------------+-----------------------------+
2 rows in set (0.00 sec)
范例： 记录去重
MariaDB [hellodb]> select distinct gender from students ;
+--------+
| gender |
+--------+
| M |
| F |
+--------+
2 rows in set (0.001 sec)
// 将 age 和 gender 多个字段重复的记录去重
mysql> select distinct age,gender from students;
范例： SQL 注入攻击
// 创建表
create table user (id int primary key auto_increment ,name varchar(20) not null ,password varchar(30) not null );
// 查看表结构
desc user;
// 插入数据
insert user values(null,'admin','123456');
insert user values(null,'wangj','456789');
// 查询表内容
select * from user;
// 类比登录认证
select * from user where name='admin' and password='123456'; # 可以查询到结果代表账户密码正确.( 登录成功 )
// 实际上我们用如下方法也可以查询到内容 ( 类似 SQL 注入 )
// 为什么可以成功 ( 因为该 SQL 语句增加了一个 or 或者 1=1 )
// 1=1 成立, 就可以查询到数据 ( 黑客就可以使用该 SQL 注入方法基于任何身份登录系统 )
select * from user where name='admin' and password='' or '1=1';
select * from user where name='admin' and password='' or '1'='1';
// 包括如下方法也可以实现 ( -- 为注释 )
select * from user where name='admin'; -- ' and password='xxxxxx';
select * from user where name='admin'; # ' and password='xxxxx';
范例：分页查询
# 只取前 3 个
mysql> select * from students limit 0,3;
mysql> select * from students limit 3;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
+-------+-------------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)
mysql> select * from students limit 1,3;
+-------+------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+------------+-----+--------+---------+-----------+
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
+-------+------------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)
# 查询第 n 页的数据, 每页显示 m 条记录
mysql> select * from students limit (n-1) * m,m;
范例：聚合函数
mysql> select sum(age)/count(*) from students where gender ='M';
+-------------------+
| sum(age)/count(*) |
+-------------------+
| 33.0000 |
+-------------------+
1 row in set (0.00 sec)
mysql> select sum(age)/count(*) from students where gender ='F';
+-------------------+
| sum(age)/count(*) |
+-------------------+
| 19.0000 |
+-------------------+
1 row in set (0.00 sec)
范例： 分组统计
// 统计每个 classid 出现的数量
mysql> select classid 班级,count(*) 学员数量 from students group by classid;
+--------+--------------+
| 班级 | 学员数量 |
+--------+--------------+
| NULL | 2 |
| 1 | 4 |
| 2 | 3 |
| 3 | 4 |
| 4 | 4 |
| 5 | 1 |
| 6 | 4 |
| 7 | 3 |
+--------+--------------+
8 rows in set (0.00 sec)
MariaDB [hellodb]> select classid 班级,gender 性别,count(*) 数量 from students group by classid,gender;
+--------+--------+--------+
| 班级 | 性别 | 数量 |
+--------+--------+--------+
| NULL | M | 2 |
| 1 | F | 2 |
| 1 | M | 2 |
| 2 | M | 3 |
| 3 | F | 3 |
| 3 | M | 1 |
| 4 | M | 4 |
| 5 | M | 1 |
| 6 | F | 3 |
| 6 | M | 1 |
| 7 | F | 2 |
| 7 | M | 1 |
+--------+--------+--------+
12 rows in set (0.00 sec)
// 分组统计
select classid,avg(age) as 平均年龄 from students where classid > 3 group by classid having 平均年龄 >30 ;
select gender,avg(age) 平均年龄 from students group by gender having gender='M';
// 多个字段分组统计
select classid,gender,count(*) 数量 from students group by classid,gender;
select classid,gender,count(*) 数量 from students group by gender,classid;
// 平均值
MariaDB [hellodb]> select gender 性别,avg(age) 平均年龄 from students group by gender;
+--------+--------------+
| 性别 | 平均年龄 |
+--------+--------------+
| F | 19.0000 |
| M | 33.0000 |
+--------+--------------+
2 rows in set (0.00 sec)
**范例：**group_concat 函数实现分组信息的集合
mysql> select gender,group_concat(name) from students group by gender;
范例： 分组统计
// with rollup 分组后聚合函数统计后再做汇总
mysql> select gender,count(*) from students group by gender with rollup;
+--------+----------+
| gender | count(*) |
+--------+----------+
| F | 10 |
| M | 15 |
| NULL | 25 |
+--------+----------+
3 rows in set (0.00 sec)
mysql> select gender,group_concat(name) from students group by gender with rollup;
范例： 分组统计
// 注意: 一旦使用分组 group by
// 我们在 select 后面的只跟 采用分组的列 或 聚合函数
// 不要将其它的列放在 select
后面, 否则根据系统变量 SQL_MODE 的值不同而不同的结果
// 以下为 MySQL8.0.17 的执行结果
mysql> use hellodb
mysql> select classid,count(*) 数量 from students group by classid;
+---------+--------+
| classid | 数量 |
+---------+--------+
| 2 | 3 |
| 1 | 4 |
| 4 | 4 |
| 3 | 4 |
| 5 | 1 |
| 7 | 3 |
| 6 | 4 |
| NULL | 2 |
+---------+--------+
8 rows in set (0.00 sec)
mysql> select classid,count(*),stuid 数量 from students group by classid;
mysql> select @@sql_mode;
// 以下是 Mariadb10.3.17 的执行结果
MariaDB [hellodb]> select classid, count(*), stuid from students group by classid;
+---------+----------+-------+
| classid | count(*) | stuid |
+---------+----------+-------+
| NULL | 2 | 24 |
| 1 | 4 | 2 |
| 2 | 3 | 1 |
| 3 | 4 | 5 |
| 4 | 4 | 4 |
| 5 | 1 | 6 |
| 6 | 4 | 9 |
| 7 | 3 | 8 |
+---------+----------+-------+
8 rows in set (0.001 sec)
MariaDB [hellodb]> select @@sql_mode;
范例： 排序
// 只取前 3 个
mysql> select * from students order by age desc limit 3;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
+-------+-------------+-----+--------+---------+-----------+
3 rows in set (0.00 sec)
// 跳过前 3 个只显示后续的 2 个
mysql> select * from students order by age desc limit 3,2;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
+-------+--------------+-----+--------+---------+-----------+
2 rows in set (0.00 sec)
// 排序 ( 默认从小到大 )
select * from students order by age;
// 倒序
select * from students order by age desc;
范例： 排序
select classid,sum(age) from students where classid is not null group by
classid order by classid;
select classid,sum(age) from students group by classid having classid is not null order by classid;
select classid,sum(age) from students where classid is not null group by
classid order by classid limit 2,3;
// 必须先过滤, 再排序
select * from students where classid is not null order by gender desc, age asc;
// 多列排序
select * from students order by gender desc, age asc;
**范例：**正序排序时将 NULL 记录排在最后
// 对 classid 正序排序, NULL 记录排在最后
select * from students order by -classid desc;
范例： 分组和排序
mysql> select classid,count(*) 数量 from students group by classid order by 数量;
+---------+--------+
| classid | 数量 |
+---------+--------+
| 5 | 1 |
| NULL | 2 |
| 2 | 3 |
| 7 | 3 |
| 1 | 4 |
| 4 | 4 |
| 3 | 4 |
| 6 | 4 |
+---------+--------+
8 rows in set (0.00 sec)
# 分组后再排序
MariaDB [hellodb]> select gender,classid,avg(age) from students where classid is not null group by gender,classid order by gender,classid;
+--------+---------+----------+
| gender | classid | avg(age) |
+--------+---------+----------+
| F | 1 | 19.0000 |
| F | 3 | 18.3333 |
| F | 6 | 20.0000 |
| F | 7 | 18.0000 |
| F | 77 | 18.0000 |
| F | 93 | 18.0000 |
| M | 1 | 21.5000 |
| M | 2 | 35.2000 |
| M | 3 | 23.0000 |
| M | 4 | 23.6000 |
| M | 5 | 46.0000 |
| M | 6 | 23.0000 |
| M | 7 | 23.0000 |
| M | 94 | 18.0000 |
+--------+---------+----------+
14 rows in set (0.001 sec)
MariaDB [hellodb]> select * from students order by age limit 10;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 33 | Miejue Shitai | 18 | F | 77 | NULL |
| 32 | Zhang Sanfeng | 18 | M | 94 | NULL |
| 27 | liudehua | 18 | F | 1 | NULL |
| 34 | Lin Chaoying | 18 | F | 93 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
+-------+---------------+-----+--------+---------+-----------+
10 rows in set (0.001 sec)
MariaDB [hellodb]> select * from students order by age limit 3,10;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 34 | Lin Chaoying | 18 | F | 93 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 32 | Zhang Sanfeng | 18 | M | 94 | NULL |
| 27 | liudehua | 18 | F | 1 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 29 | wuyanzu | 19 | M | 4 | NULL |
+-------+---------------+-----+--------+---------+-----------+
10 rows in set (0.000 sec)
MariaDB [hellodb]> select distinct age from students order by age limit 3;
+-----+
| age |
+-----+
| 17 |
| 18 |
| 19 |
+-----+
3 rows in set (0.001 sec)
MariaDB [hellodb]> select distinct age from students order by age limit 3,5;
+-----+
| age |
+-----+
| 20 |
| 21 |
| 22 |
| 23 |
| 25 |
+-----+
5 rows in set (0.001 sec)
范例： 分组和排序的次序
// 顺序: group by,having,order by
mysql> select classid,count(*) from students group by classid having classid is not null order by classid;
+---------+----------+
| classid | count(*) |
+---------+----------+
| 1 | 4 |
| 2 | 3 |
| 3 | 4 |
| 4 | 4 |
| 5 | 1 |
| 6 | 4 |
| 7 | 3 |
+---------+----------+
7 rows in set (0.00 sec)
// 以下顺序会出错, group by,order by,having
mysql> select classid,count(*) from students group by classid order by classid having classid is not null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'having classid is not null' at line 1
// 以下顺序会出错, order by,group by,having
mysql> select classid,count(*) from students order by classid group by classid having classid is not null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'group by classid having classid is not null' at line 1
范例： 时间字段进行过滤查询，并且 timestamp 可以随其它字段的更新自动更新
MariaDB [testdb]> create table testdate (id int auto_increment primary key,date timestamp DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP);
MariaDB [testdb]> insert testdate () values();
MariaDB [testdb]> insert testdate values(),(),();
MariaDB [testdb]> select * from testdate;
+----+---------------------+
| id | date |
+----+---------------------+
| 1 | 2020-06-03 15:21:03 |
| 2 | 2020-06-03 15:21:12 |
| 3 | 2020-06-03 15:21:14 |
| 4 | 2020-06-03 15:21:17 |
| 5 | 2020-06-03 18:27:39 |
| 6 | 2020-06-03 18:27:44 |
+----+---------------------+
6 rows in set (0.001 sec)
MariaDB [testdb]> select * from testdate where date between '2020-06-03 15:21:12' and '2020-06-03 18:27:40';
+----+---------------------+
| id | date |
+----+---------------------+
| 2 | 2020-06-03 15:21:12 |
| 3 | 2020-06-03 15:21:14 |
| 4 | 2020-06-03 15:21:17 |
| 5 | 2020-06-03 18:27:39 |
+----+---------------------+
4 rows in set (0.000 sec)
MariaDB [testdb]> select * from testdate where date >= '2020-06-03 15:21:12' and date <= '2020-06-03 18:27:40';
+----+---------------------+
| id | date |
+----+---------------------+
| 2 | 2020-06-03 15:21:12 |
| 3 | 2020-06-03 15:21:14 |
| 4 | 2020-06-03 15:21:17 |
| 5 | 2020-06-03 18:27:39 |
+----+---------------------+
4 rows in set (0.001 sec)
# 修改其它字段, 会自动更新 timestamp 字段
mysql> update testdate set id=10 where id=1;
mysql> select * from testdate3;
+----+---------------------+
| id | date |
+----+---------------------+
| 2 | 2020-06-03 15:21:12 |
| 3 | 2020-06-03 15:21:14 |
| 4 | 2020-06-03 15:21:17 |
| 5 | 2020-06-03 18:27:39 |
| 6 | 2020-06-03 18:27:44 |
| 10 | 2020-06-03 18:34:51 |
+----+---------------------+
6 rows in set (0.001 sec)
3.7.2） 多表查询
多表查询 ，即查询结果来自于多张表
**子查询：**在SQL语句嵌套着查询语句，性能较差，基于某语句的查询结果再次进行的查询
**联合查询：**UNION
**交叉连接：**笛卡尔乘积 CROSS JOIN
内连接：
等值连接：让表之间的字段以"等值"建立连接关系
不等值连接
自然连接：去掉重复列的等值连接 , 语法: FROM table1 NATURAL JOIN table2;
外连接：
**左外连接：**FROM tb1 LEFT JOIN tb2 ON tb1.col=tb2.col
**右外连接：**FROM tb1 RIGHT JOIN tb2 ON tb1.col=tb2.col
完全外连接: FROM tb1 FULL OUTER JOIN tb2 ON tb1.col=tb2.col 注意:MySQL 不支持此 SQL 语法
**自连接：**本表和本表进行连接查询
3.7.2.1）子查询
子查询 subquery 即 SQL语句 调用另一个 SELECT 子句，可以是对同一张表，也可以是对不同表，主要有以下四种常见的用法。
// 举例
select * from students where age > (select avg(age) from students);
// 举例
update students set age= (select avg(age) from students) where stuid=25;
update teachers set age= (select avg(age) from students) where tid=4;
用于比较表达式中的子查询；子查询仅能返回单个值
SELECT Name,Age FROM students WHERE Age>(SELECT avg(Age) FROM teachers);
update students set Age=(SELECT avg(Age) FROM teachers) where stuid=25;
用于 IN 中的子查询：子查询应该单独查询并返回一个或多个值重新构成列表
SELECT Name,Age FROM students WHERE Age IN (SELECT Age FROM teachers);
用于EXISTS 和 Not EXISTS
参考链接： MySQL :: MySQL 8.0 Reference Manual :: 13.2.15.6 Subqueries with EXISTS or NOT EXISTS
EXISTS（包括 NOT EXISTS）子句的返回值是一个 BOOL 值。
EXISTS 内部有一个子查询语句（SELECT ... FROM...），将其称为 EXIST 的内查询语句。
其内查询语句返回一个结果集。 EXISTS 子句根据其内查询语句的结果集空或者非空，返回一个布尔值。
将外查询表的每一行，代入内查询作为检验，如果内查询返回的结果为非空值，则 EXISTS 子句返回 TRUE，外查询的这一行数据便可作为外查询的结果行返回，否则不能作为结果。
MariaDB [hellodb]> select * from students s where EXISTS (select * from teachers t where s.teacherid=t.tid);
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
+-------+-------------+-----+--------+---------+-----------+
# 说明：
1) EXISTS (或 NOT EXISTS) 用在 where 之后, 且后面紧跟子查询语句 (带括号)
2) EXISTS (或 NOT EXISTS) 只关心子查询有没有结果, 并不关心子查询的结果具体是什么
3) 上述语句把 students 的记录逐条代入到 Exists 后面的子查询中, 如果子查询结果集不为空, 即说明
存在, 那么这条 students 的记录出现在最终结果集, 否则被排除.
MariaDB [hellodb]> select * from students s where NOT EXISTS (select * from teachers t where s.teacherid=t.tid);
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
22 rows in set (0.001 sec)
用于 FROM 子句中的子查询
使用格式：
SELECT tb_alias.col1,... FROM (SELECT clause) AS tb_alias WHERE Clause;
范例：
SELECT s.ClassID,s.aage FROM (SELECT ClassID,avg(Age) AS aage FROM students WHERE ClassID IS NOT NULL GROUP BY ClassID) AS s WHERE s.aage>30;
范例： 子查询
# 子查询: select 的执行结果, 被其它 SQL 调用
MariaDB [hellodb]> select stuid,name,age from students where age > (select avg(age) from students);
+-------+--------------+-----+
| stuid | name | age |
+-------+--------------+-----+
| 3 | Xie Yanke | 53 |
| 4 | Ding Dian | 32 |
| 6 | Shi Qing | 46 |
| 13 | Tian Boguang | 33 |
| 25 | Sun Dasheng | 100 |
+-------+--------------+-----+
5 rows in set (0.00 sec)
范例： 子查询用于更新表
MariaDB [hellodb]> update teachers set age=(select avg(age) from students) where tid=4;
Query OK, 1 row affected (0.00 sec)
MariaDB [hellodb]> select * from teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 27 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)
3.7.2.2）联合查询
联合查询 Union 实现的条件，多个表的字段数量相同，字段名和数据类型可以不同，但一般数据类型是相同的。
SELECT Name,Age FROM students UNION SELECT Name,Age FROM teachers;
范例： 联合查询
// 多表纵向合并 union
MariaDB [hellodb]> select * from teachers union select * from students;
MariaDB [hellodb]> select tid as id,name,age,gender from teachers union select stuid,name,age,gender from students;
+----+---------------+-----+--------+
| id | name | age | gender |
+----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 26 | F |
| 1 | Shi Zhongyu | 22 | M |
| 2 | Shi Potian | 22 | M |
| 3 | Xie Yanke | 53 | M |
| 4 | Ding Dian | 32 | M |
| 5 | Yu Yutong | 26 | M |
| 6 | Shi Qing | 46 | M |
| 7 | Xi Ren | 19 | F |
| 8 | Lin Daiyu | 17 | F |
| 9 | Ren Yingying | 20 | F |
| 10 | Yue Lingshan | 19 | F |
| 11 | Yuan Chengzhi | 23 | M |
| 12 | Wen Qingqing | 19 | F |
| 13 | Tian Boguang | 33 | M |
| 14 | Lu Wushuang | 17 | F |
| 15 | Duan Yu | 19 | M |
| 16 | Xu Zhu | 21 | M |
| 17 | Lin Chong | 25 | M |
| 18 | Hua Rong | 23 | M |
| 19 | Xue Baochai | 18 | F |
| 20 | Diao Chan | 19 | F |
| 21 | Huang Yueying | 22 | F |
| 22 | Xiao Qiao | 20 | F |
| 23 | Ma Chao | 23 | M |
| 24 | Xu Xian | 27 | M |
| 25 | Sun Dasheng | 100 | M |
| 26 | xietingfeng | 23 | M |
| 27 | liudehua | 18 | F |
| 28 | mahuateng | 20 | M |
| 29 | wuyanzu | 19 | M |
| 30 | wuxin | 21 | M |
| 31 | Song Jiang | 45 | M |
| 32 | Zhang Sanfeng | 18 | M |
| 33 | Miejue Shitai | 18 | F |
| 34 | Lin Chaoying | 18 | F |
| 35 | 巴西可 | 20 | M |
| 36 | abc | 20 | M |
+----+---------------+-----+--------+
40 rows in set (0.001 sec)
MariaDB [hellodb]> select * from teachers union select * from teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)
MariaDB [hellodb]> select * from teachers union all select * from teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
MariaDB [hellodb]> select * from user union select * from user;
+------+----------+----------+
| id | username | password |
+------+----------+----------+
| 1 | admin | magedu |
| 2 | mage | magedu |
| 3 | wang | centos |
+------+----------+----------+
3 rows in set (0.00 sec)
MariaDB [hellodb]> select distinct * from user ;
+------+----------+----------+
| id | username | password |
+------+----------+----------+
| 1 | admin | magedu |
| 2 | mage | magedu |
| 3 | wang | centos |
+------+----------+----------+
3 rows in set (0.00 sec)
范例： 去重记录
mysql> select * from emp;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | mage | 20 | M |
| 4 | li | 22 | F |
| 3 | mage | 20 | M |
+-----+---------------+-----+--------+
5 rows in set (0.00 sec)
mysql> select distinct * from emp;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | mage | 20 | M |
| 4 | li | 22 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)
mysql> select * from emp union select * from emp;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | mage | 20 | M |
| 4 | li | 22 | F |
+-----+---------------+-----+--------+
4 rows in set (0.00 sec)
# union all 不去重
mysql> select * from emp union all select * from emp;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | mage | 20 | M |
| 4 | li | 22 | F |
| 3 | mage | 20 | M |
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | mage | 20 | M |
| 4 | li | 22 | F |
| 3 | mage | 20 | M |
+-----+---------------+-----+--------+
10 rows in set (0.00 sec)
3.7.2.3） 交叉连接
cross join 即多表的记录之间做笛卡尔乘积组合，并且多个表的列横向合并相加，"雨露均沾"
**比如：**第一个表 3 行 4 列，第二个表 5 行 6 列，cross join 后的结果为 3*5 = 15 行，4+6 = 10列
交叉连接生成的记录可能会非常多，建议慎用
**范例：**交叉连接
// 横向合并, 交叉连接 (横向笛卡尔)
MariaDB [hellodb]> select * from students cross join teachers;
MariaDB [hellodb]> select * from teachers , students;
MariaDB [hellodb]> select stuid,students.name student_name,students.age,tid,teachers.name teacher_name,teachers.age from teachers cross join students;
MariaDB [hellodb]> select stuid,s.name student_name,s.age student_age,tid,t.name teacher_name,t.age teacher_age from teachers t cross join students s ;
3.7.2.4）内连接
inner join 内连接取多个表的交集
范例： 内连接
// 内连接 inner join
MariaDB [hellodb]> select * from students inner join teachers on students.teacherid=teachers.tid;
// 表定义别名
// 如果表定义了别名, 原表名将无法使用
MariaDB [hellodb]> select stuid,s.name as student_name ,tid,t.name as teacher_name from students as s inner join teachers as t on s.teacherid=t.tid;
+-------+--------------+-----+---------------+
| stuid | student_name | tid | teacher_name |
+-------+--------------+-----+---------------+
| 5 | Yu Yutong | 1 | Song Jiang |
| 1 | Shi Zhongyu | 3 | Miejue Shitai |
| 4 | Ding Dian | 4 | Lin Chaoying |
+-------+--------------+-----+---------------+
3 rows in set (0.00 sec)
MariaDB [hellodb]> select stuid,s.name studentname,s.age studentage,tid,t.name as teachername,t.age teacherage from students as s inner join teachers t on s.teacherid=t.tid;
+-------+-------------+------------+-----+---------------+------------+
| stuid | studentname | studentage | tid | teachername | teacherage |
+-------+-------------+------------+-----+---------------+------------+
| 5 | Yu Yutong | 26 | 1 | Song Jiang | 45 |
| 25 | Sun Dasheng | 100 | 1 | Song Jiang | 45 |
| 1 | Shi Zhongyu | 22 | 3 | Miejue Shitai | 77 |
| 4 | Ding Dian | 32 | 4 | Lin Chaoying | 93 |
+-------+-------------+------------+-----+---------------+------------+
4 rows in set (0.00 sec)
MariaDB [hellodb]> select * from students , teachers where students.teacherid=teachers.tid;
MariaDB [hellodb]> select s.name 学生姓名,s.age 学生年龄,s.gender 学生性别,t.name 老师姓名,t.age 老师年龄,t.gender 老师性别 from students s inner join teachers t on s.gender <> t.gender;
MariaDB [hellodb]> select stuid,s.name,tid,t.name from students s,teachers t where s.teacherid=t.tid;
+-------+-------------+-----+---------------+
| stuid | name | tid | name |
+-------+-------------+-----+---------------+
| 5 | Yu Yutong | 1 | Song Jiang |
| 1 | Shi Zhongyu | 3 | Miejue Shitai |
| 4 | Ding Dian | 4 | Lin Chaoying |
+-------+-------------+-----+---------------+
3 rows in set (0.00 sec)
// 内连接后过滤数据
MariaDB [hellodb]> select * from students s inner join teachers t on s.teacherid=t.tid and s.age > 30 ;
MariaDB [hellodb]> select * from students s inner join teachers t on
s.teacherid=t.tid where s.age > 30 ;
自然连接
当源表和目标表共享相同名称的列时，就可以在它们之间执行自然连接，而无需指定连接列。
在使用纯自然连接时，如没有相同的列时，会产生交叉连接（笛卡尔乘积）
语法： (SQL:1999)SELECT table1.column, table2.column FROM table1 NATURAL JOIN table2;
范例：
MariaDB [db1]> create table t1 ( id int,name char(20));
MariaDB [db1]> create table t2 ( id int,title char(20));
MariaDB [db1]> insert t1 values(1,'mage'),(2,'wang'),(3,'zhang');
MariaDB [db1]> insert t2 values(1,'ceo'),(2,'cto');
MariaDB [db1]> select * from t1;
+------+-------+
| id | name |
+------+-------+
| 1 | mage |
| 2 | wang |
| 3 | zhang |
+------+-------+
3 rows in set (0.00 sec)
MariaDB [db1]> select * from t2;
+------+-------+
| id | title |
+------+-------+
| 1 | ceo |
| 2 | cto |
+------+-------+
2 rows in set (0.00 sec)
MariaDB [db1]> select * from t1 NATURAL JOIN t2;
+------+------+-------+
| id | name | title |
+------+------+-------+
| 1 | mage | ceo |
| 2 | wang | cto |
+------+------+-------+
2 rows in set (0.00 sec)
MariaDB [db1]> select t1.name,t2.title from t1 NATURAL JOIN t2;
+------+-------+
| name | title |
+------+-------+
| mage | ceo |
| wang | cto |
+------+-------+
2 rows in set (0.00 sec)
3.7.2.4）左和右外连接
**左连接：**以左表为主根据条件查询右表数据，如果根据条件查询右表数据不存在使用 null 值填充
**右连接：**以右表为主根据条件查询左表数据，如果根据条件查询左表数据不存在使用 null 值填充
范例： 左，右外连接
// 左外连接 left outer join
MariaDB [hellodb]> select s.stuid,s.name,s.age,s.teacherid,t.tid,t.name,t.age from students as s left outer join teachers as t on s.teacherid=t.tid;
+-------+---------------+-----+-----------+------+---------------+------+
| stuid | name | age | teacherid | tid | name | age |
+-------+---------------+-----+-----------+------+---------------+------+
| 1 | Shi Zhongyu | 22 | 3 | 3 | Miejue Shitai | 77 |
| 2 | Shi Potian | 22 | 7 | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | 16 | NULL | NULL | NULL |
| 4 | Ding Dian | 32 | 4 | 4 | Lin Chaoying | 93 |
| 5 | Yu Yutong | 26 | 1 | 1 | Song Jiang | 45 |
| 6 | Shi Qing | 46 | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | NULL | NULL | NULL | NULL |
+-------+---------------+-----+-----------+------+---------------+------+
25 rows in set (0.00 sec)
// 左外连接扩展
MariaDB [hellodb]> select * from students s left outer join teachers t on s.teacherid=t.tid where t.tid is null;
// 多个条件的左外连接
MariaDB [hellodb]> select * from students s left outer join teachers t on s.teacherid=t.tid and s.teacherid is null;
// 先左外连接, 再过滤
MariaDB [hellodb]> select * from students s left outer join teachers t on s.teacherid=t.tid where s.teacherid is null;
// 右外连接
MariaDB [hellodb]> select * from students s right outer join teachers t on s.teacherid=t.tid;
// 右外连接的扩展用法
MariaDB [hellodb]> select * from students s right outer join teachers t on s.teacherid=t.tid where s.teacherid is null;
3.7.2.5）完全外连接
MySQL 不支持完全外连接 full outer join 语法
范例： 完全外连接
# MySQL不支持完全外连接 full outer join, 利用以下方式法代替
MariaDB [hellodb]> select * from students left join teachers on students.teacherid=teachers.tid
-> union
-> select * from students right join teachers on students.teacherid=teachers.tid;
MariaDB [hellodb]> select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s left join teachers as t on s.teacherid=t.tid
-> union
-> select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s right join teachers as t on s.teacherid=t.tid;
+-------+---------------+------+------+---------------+------+
| stuid | name | age | tid | name | age |
+-------+---------------+------+------+---------------+------+
| 1 | Shi Zhongyu | 22 | 3 | Miejue Shitai | 77 |
| 2 | Shi Potian | 22 | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | NULL | NULL | NULL |
| 4 | Ding Dian | 32 | 4 | Lin Chaoying | 93 |
| 5 | Yu Yutong | 26 | 1 | Song Jiang | 45 |
| 6 | Shi Qing | 46 | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | NULL | NULL | NULL |
| NULL | NULL | NULL | 2 | Zhang Sanfeng | 94 |
+-------+---------------+------+------+---------------+------+
26 rows in set (0.01 sec)
# 完全外连接的扩展示例
MariaDB [hellodb]> select * from students s left outer join teachers t on s.teacherid=t.tid where t.tid is null union select * from students s right outer join teachers t on s.teacherid=t.tid where s.teacherid is null;
MariaDB [hellodb]> select * from (select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s left outer join teachers t on s.teacherid=t.tid union select s.stuid,s.name,s.teacherid,t.tid,t.name from students s right outer join teachers t on s.teacherid=t.tid) as a where a.teacherid is null or a.tid is null;
+-------+---------------+-----------+------+---------------+
| stuid | s_name | teacherid | tid | t_name |
+-------+---------------+-----------+------+---------------+
| 2 | Shi Potian | 7 | NULL | NULL |
| 3 | Xie Yanke | 16 | NULL | NULL |
| 6 | Shi Qing | NULL | NULL | NULL |
| 7 | Xi Ren | NULL | NULL | NULL |
| 8 | Lin Daiyu | NULL | NULL | NULL |
| 9 | Ren Yingying | NULL | NULL | NULL |
| 10 | Yue Lingshan | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | NULL | NULL | NULL |
| 12 | Wen Qingqing | NULL | NULL | NULL |
| 13 | Tian Boguang | NULL | NULL | NULL |
| 14 | Lu Wushuang | NULL | NULL | NULL |
| 15 | Duan Yu | NULL | NULL | NULL |
| 16 | Xu Zhu | NULL | NULL | NULL |
| 17 | Lin Chong | NULL | NULL | NULL |
| 18 | Hua Rong | NULL | NULL | NULL |
| 19 | Xue Baochai | NULL | NULL | NULL |
| 20 | Diao Chan | NULL | NULL | NULL |
| 21 | Huang Yueying | NULL | NULL | NULL |
| 22 | Xiao Qiao | NULL | NULL | NULL |
| 23 | Ma Chao | NULL | NULL | NULL |
| 24 | Xu Xian | NULL | NULL | NULL |
| NULL | NULL | NULL | 2 | Zhang Sanfeng |
| NULL | NULL | NULL | 5 | abc |
+-------+---------------+-----------+------+---------------+
23 rows in set (0.00 sec)
3.7.2.6）自连接
自连接，即表自身连接自身
范例： 自连接
# 自连接
MariaDB [hellodb]> select * from emp;
+------+----------+----------+
| id | name | leaderid |
+------+----------+----------+
| 1 | mage | NULL |
| 2 | zhangsir | 1 |
| 3 | wang | 2 |
| 4 | zhang | 3 |
+------+----------+----------+
4 rows in set (0.00 sec)
MariaDB [hellodb]> select e.name,l.name from emp as e inner join emp as l on e.leaderid=l.id;
+----------+----------+
| name | name |
+----------+----------+
| zhangsir | mage |
| wang | zhangsir |
| zhang | wang |
+----------+----------+
3 rows in set (0.00 sec)
MariaDB [hellodb]> select e.name,IFNULL(l.name,'无上级') from emp as e left join emp as l on e.leaderid=l.id;
+----------+----------+
| name | name |
+----------+----------+
| zhangsir | mage |
| wang | zhangsir |
| zhang | wang |
| mage | NULL |
+----------+----------+
4 rows in set (0.00 sec)
MariaDB [hellodb]> select e.name emp,IFNULL(l.name,'无上级') leader from emp as e left join emp as l on e.leaderid=l.id;
+----------+----------+
| emp | leader |
+----------+----------+
| zhangsir | mage |
| wang | zhangsir |
| zhang | wang |
| mage | NULL |
+----------+----------+
4 rows in set (0.000 sec)
范例： 三表连接
// 三张表连接示例
MariaDB [hellodb]> select name,course,score from students st inner join scores sc on st.stuid=sc.stuid inner join courses co on sc.courseid=co.CourseID;
+-------------+----------------+-------+
| name | course | score |
+-------------+----------------+-------+
| Shi Zhongyu | Kuihua Baodian | 77 |
| Shi Zhongyu | Weituo Zhang | 93 |
| Shi Potian | Kuihua Baodian | 47 |
| Shi Potian | Daiyu Zanghua | 97 |
| Xie Yanke | Kuihua Baodian | 88 |
| Xie Yanke | Weituo Zhang | 75 |
| Ding Dian | Daiyu Zanghua | 71 |
| Ding Dian | Kuihua Baodian | 89 |
| Yu Yutong | Hamo Gong | 39 |
| Yu Yutong | Dagou Bangfa | 63 |
| Shi Qing | Hamo Gong | 96 |
| Xi Ren | Hamo Gong | 86 |
| Xi Ren | Dagou Bangfa | 83 |
| Lin Daiyu | Taiji Quan | 57 |
| Lin Daiyu | Jinshe Jianfa | 93 |
+-------------+----------------+-------+
15 rows in set (0.000 sec)
MariaDB [hellodb]> select st.name,co.Course,sc.score from courses co inner join scores sc on co.courseid=sc.courseid inner join students st on
sc.stuid=st.stuid;
+-------------+----------------+-------+
| name | Course | score |
+-------------+----------------+-------+
| Shi Zhongyu | Kuihua Baodian | 77 |
| Shi Zhongyu | Weituo Zhang | 93 |
| Shi Potian | Kuihua Baodian | 47 |
| Shi Potian | Daiyu Zanghua | 97 |
| Xie Yanke | Kuihua Baodian | 88 |
| Xie Yanke | Weituo Zhang | 75 |
| Ding Dian | Daiyu Zanghua | 71 |
| Ding Dian | Kuihua Baodian | 89 |
| Yu Yutong | Hamo Gong | 39 |
| Yu Yutong | Dagou Bangfa | 63 |
| Shi Qing | Hamo Gong | 96 |
| Xi Ren | Hamo Gong | 86 |
| Xi Ren | Dagou Bangfa | 83 |
| Lin Daiyu | Taiji Quan | 57 |
| Lin Daiyu | Jinshe Jianfa | 93 |
+-------------+----------------+-------+
15 rows in set (0.001 sec)
3.7.3）SELECT 语句处理的顺序
查询执行路径中的组件：查询缓存、解析器、预处理器、优化器、查询执行引擎、存储引擎
SELECT 语句的执行流程：
FROM Clause --> WHERE Clause --> GROUP BY --> HAVING Clause -->SELECT --> ORDER BY --> LIMIT
练习题：（ 重要 ）
导入 hellodb.sql 生成数据库
在 students 表中，查询年龄大于25岁，且为男性的同学的名字和年龄
SELECT name,age FROM students WHERE age > 25 AND gender = 'M';
以 ClassID 字段 为分组依据，显示各组的平均年龄
SELECT ClassID,AVG(age) AS avg_age
FROM students
GROUP BY ClassID;
显示第 2 题中平均年龄大于 30 的分组及平均年龄
要显示第 2 题中平均年龄大于 30 的分组及其平均年龄，您可以使用第 2 题的查询作为子查询，并在外部查询中筛选出平均年龄大于 30 的分组。
SELECT ClassID, avg_age
FROM (
SELECT ClassID, AVG(age) AS avg_age
FROM students
GROUP BY ClassID
) AS avg_per_group
WHERE avg_age > 30;
显示以 L 开头的名字的同学的信息
SELECT * FROM students
WHERE name LIKE 'L%';
显示 TeacherID 非空的同学的相关信息
SELECT * FROM students
WHERE TeacherID IS NOT NULL;
以年龄排序后，显示年龄最大的前 10 位同学的信息
SELECT * FROM students
ORDER BY age DESC
LIMIT 10;
查询年龄大于等于 20 岁，小于等于 25 岁的同学的信息
SELECT * FROM students WHERE age >= 20 AND age <= 25;
以 ClassID 分组，显示每班的同学的人数
SELECT ClassID, COUNT(*) AS student_count
FROM students
GROUP BY ClassID;
以 Gender 分组，显示其年龄之和
SELECT Gender, SUM(age) AS total_age
FROM students
GROUP BY Gender;
以 ClassID 分组，显示其平均年龄大于 25 的班级
SELECT ClassID, AVG(age) AS avg_age
FROM students
GROUP BY ClassID
HAVING AVG(age) > 25;
以 Gender 分组，显示各组中年龄大于 25 的学员的年龄之和
SELECT Gender, SUM(age) AS total_age
FROM students
GROUP BY Gender
HAVING SUM(CASE WHEN age > 25 THEN age ELSE 0 END) > 0;
显示前 5 位同学的姓名、课程及成绩
显示其成绩高于 80 的同学的名称及课程
取每位同学各门课的平均成绩，显示成绩前三名的同学的姓名和平均成绩
显示每门课程课程名称及学习了这门课的同学的个数
显示其年龄大于平均年龄的同学的名字
显示其学习的课程为第 1、2，4 或第 7 门课的同学的名字
显示其成员数最少为 3 个的班级的同学中年龄大于同班同学平均年龄的同学
统计各班级中年龄大于全校同学平均年龄的同学
扩展 ：（ 多表查询 ）
// 多表查询 ( "两张表查询" )
MariaDB [hellodb]> select st.name,sc.courseid,sc.score from students st inner join scores sc on st.stuid=sc.stuid;
+-------------+----------+-------+
| name | courseid | score |
+-------------+----------+-------+
| Shi Zhongyu | 2 | 77 |
| Shi Zhongyu | 6 | 93 |
| Shi Potian | 2 | 47 |
| Shi Potian | 5 | 97 |
| Xie Yanke | 2 | 88 |
| Xie Yanke | 6 | 75 |
| Ding Dian | 5 | 71 |
| Ding Dian | 2 | 89 |
| Yu Yutong | 1 | 39 |
| Yu Yutong | 7 | 63 |
| Shi Qing | 1 | 96 |
| Xi Ren | 1 | 86 |
| Xi Ren | 7 | 83 |
| Lin Daiyu | 4 | 57 |
| Lin Daiyu | 3 | 93 |
+-------------+----------+-------+
15 rows in set (0.01 sec)
// 多表查询 ( "三张表查询" )
MariaDB [hellodb]> select st.name,co.course,sc.score from students st inner join scores sc on st.stuid=sc.stuid inner join courses co on sc.courseid=co.courseid;
+-------------+----------------+-------+
| name | course | score |
+-------------+----------------+-------+
| Shi Zhongyu | Kuihua Baodian | 77 |
| Shi Zhongyu | Weituo Zhang | 93 |
| Shi Potian | Kuihua Baodian | 47 |
| Shi Potian | Daiyu Zanghua | 97 |
| Xie Yanke | Kuihua Baodian | 88 |
| Xie Yanke | Weituo Zhang | 75 |
| Ding Dian | Daiyu Zanghua | 71 |
| Ding Dian | Kuihua Baodian | 89 |
| Yu Yutong | Hamo Gong | 39 |
| Yu Yutong | Dagou Bangfa | 63 |
| Shi Qing | Hamo Gong | 96 |
| Xi Ren | Hamo Gong | 86 |
| Xi Ren | Dagou Bangfa | 83 |
| Lin Daiyu | Taiji Quan | 57 |
| Lin Daiyu | Jinshe Jianfa | 93 |
+-------------+----------------+-------+
15 rows in set (0.00 sec)
