【MySQL】 基本查询(下)

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本篇主题 ：【MySQL】 基本查询(下)
发布时间 ：2025.2.18
隶属专栏 ：MySQL

目录

• Update
• • 语法
  • 案例
• Delete
• • 删除数据
  • • 语法
    • 案例
  • 截断表
  • • 语法
    • 案例
• 插入查询结果
• • 语法
  • 案例
• 聚合函数
• • 函数介绍
  • 案例
• [group by子句的使用](#group by子句的使用)
• • 语法
  • having和where
  • 案例
• 结语

Update

语法

UPDATE table_name SET column = expr [, column = expr ...]
	[WHERE ...] [ORDER BY ...] [LIMIT ...]

对查询到的结果进行列值更新

案例

1. 将孙悟空同学的数学成绩变更为 80 分

mysql> select name, math from exam_result;
+-----------+------+
| name      | math |
+-----------+------+
| 唐三藏    |   98 |
| 孙悟空    |   78 |
| 猪悟能    |   98 |
| 曹孟德    |   84 |
| 刘玄德    |   85 |
| 孙权      |   73 |
| 宋公明    |   65 |
+-----------+------+
7 rows in set (0.00 sec)

mysql> update exam_result set math=80 where name='孙悟空';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select name, math from exam_result;
+-----------+------+
| name      | math |
+-----------+------+
| 唐三藏    |   98 |
| 孙悟空    |   80 |
| 猪悟能    |   98 |
| 曹孟德    |   84 |
| 刘玄德    |   85 |
| 孙权      |   73 |
| 宋公明    |   65 |
+-----------+------+
7 rows in set (0.00 sec)

2. 将曹孟德同学的数学成绩变更为 60 分，语文成绩变更为 70 分

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |      67 |   98 |      56 |
|  2 | 孙悟空    |      87 |   80 |      77 |
|  3 | 猪悟能    |      88 |   98 |      90 |
|  4 | 曹孟德    |      82 |   84 |      67 |
|  5 | 刘玄德    |      55 |   85 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
|  7 | 宋公明    |      75 |   65 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)

mysql> update exam_result set math=60, chinese=70  where name='曹孟德';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |      67 |   98 |      56 |
|  2 | 孙悟空    |      87 |   80 |      77 |
|  3 | 猪悟能    |      88 |   98 |      90 |
|  4 | 曹孟德    |      70 |   60 |      67 |
|  5 | 刘玄德    |      55 |   85 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
|  7 | 宋公明    |      75 |   65 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)

3. 将总成绩倒数前三的 3 位同学的数学成绩加上 30 分

mysql> select name, math+chinese+english total from exam_result order by total asc limit 3;
+-----------+-------+
| name      | total |
+-----------+-------+
| 宋公明    |   170 |
| 刘玄德    |   185 |
| 曹孟德    |   197 |
+-----------+-------+
3 rows in set (0.00 sec)

mysql> update exam_result set math=math+30 order by chinese+math+english limit 3;
Query OK, 3 rows affected (0.02 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select name, math+chinese+english total from exam_result;
+-----------+-------+
| name      | total |
+-----------+-------+
| 唐三藏    |   221 |
| 孙悟空    |   244 |
| 猪悟能    |   276 |
| 曹孟德    |   227 |
| 刘玄德    |   215 |
| 孙权      |   221 |
| 宋公明    |   200 |
+-----------+-------+
7 rows in set (0.00 sec)

4. 将所有同学的语文成绩更新为原来的 2 倍

注意：更新全表的语句慎用！

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |      67 |   98 |      56 |
|  2 | 孙悟空    |      87 |   80 |      77 |
|  3 | 猪悟能    |      88 |   98 |      90 |
|  4 | 曹孟德    |      70 |   90 |      67 |
|  5 | 刘玄德    |      55 |  115 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
|  7 | 宋公明    |      75 |   95 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)

mysql> update exam_result set chinese=chinese*2;
Query OK, 7 rows affected (0.00 sec)
Rows matched: 7  Changed: 7  Warnings: 0

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |     134 |   98 |      56 |
|  2 | 孙悟空    |     174 |   80 |      77 |
|  3 | 猪悟能    |     176 |   98 |      90 |
|  4 | 曹孟德    |     140 |   90 |      67 |
|  5 | 刘玄德    |     110 |  115 |      45 |
|  6 | 孙权      |     140 |   73 |      78 |
|  7 | 宋公明    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)

Delete

删除数据

语法

DELETE FROM table_name [WHERE ...] [ORDER BY ...] [LIMIT ...]

案例

1. 删除孙悟空同学的考试成绩

mysql> select * from exam_result where name='孙悟空';
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  2 | 孙悟空    |     174 |   80 |      77 |
+----+-----------+---------+------+---------+
1 row in set (0.00 sec)

mysql> delete from exam_result where name='孙悟空';
Query OK, 1 row affected (0.00 sec)

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |     134 |   98 |      56 |
|  3 | 猪悟能    |     176 |   98 |      90 |
|  4 | 曹孟德    |     140 |   90 |      67 |
|  5 | 刘玄德    |     110 |  115 |      45 |
|  6 | 孙权      |     140 |   73 |      78 |
|  7 | 宋公明    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
6 rows in set (0.00 sec)

2. 删除倒数第一名

mysql> select name, chinese+math+english total from exam_result order by total asc limit 1;
+-----------+-------+
| name      | total |
+-----------+-------+
| 刘玄德    |   270 |
+-----------+-------+
1 row in set (0.00 sec)

mysql> delete from exam_result order by chinese+math+english asc limit 1;
Query OK, 1 row affected (0.01 sec)

mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 唐三藏    |     134 |   98 |      56 |
|  3 | 猪悟能    |     176 |   98 |      90 |
|  4 | 曹孟德    |     140 |   90 |      67 |
|  6 | 孙权      |     140 |   73 |      78 |
|  7 | 宋公明    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
5 rows in set (0.00 sec)

3. 删除整张表数据

注意：删除整表操作要慎用！

创建表结构插入数据并查看。

mysql> CREATE TABLE for_delete (
    -> id INT PRIMARY KEY AUTO_INCREMENT,
    -> name VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from for_delete;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)

mysql> show create table for_delete\G
*************************** 1. row ***************************
       Table: for_delete
Create Table: CREATE TABLE `for_delete` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

删除数据

mysql> delete from for_delete;
Query OK, 3 rows affected (0.01 sec)

mysql> select * from for_delete;
Empty set (0.00 sec)

插入数据并查看表结构

mysql> insert into for_delete (name) values ('D');
Query OK, 1 row affected (0.00 sec)

mysql> show create table for_delete\G
*************************** 1. row ***************************
       Table: for_delete
Create Table: CREATE TABLE `for_delete` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select * from for_delete;
+----+------+
| id | name |
+----+------+
|  4 | D    |
+----+------+
1 row in set (0.00 sec)

截断表

语法

TRUNCATE [TABLE] table_name

注意：这个操作慎用

1. 只能对整表操作，不能像 DELETE 一样针对部分数据操作；
2. 实际上 MySQL 不对数据操作，所以比 DELETE 更快，但是TRUNCATE在删除数据的时候，并不经过真正的事物，所以无法回滚
3. 会重置 AUTO_INCREMENT 项

案例

创建表结构并插入数据

mysql> CREATE TABLE for_truncate (
    -> id INT PRIMARY KEY AUTO_INCREMENT,
    -> name VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from for_truncate;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)

截断整表数据，注意影响行数是 0，所以实际上没有对数据真正操作

mysql> truncate for_truncate;
Query OK, 0 rows affected (0.03 sec)

mysql> select * from for_truncate;
Empty set (0.00 sec)

再插入一条数据，自增 id 在重新增长

mysql> insert into for_truncate (name) values ('D');
Query OK, 1 row affected (0.01 sec)

mysql> show create table for_truncate\G
*************************** 1. row ***************************
       Table: for_truncate
Create Table: CREATE TABLE `for_truncate` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select * from for_truncate;
+----+------+
| id | name |
+----+------+
|  1 | D    |
+----+------+
1 row in set (0.00 sec)

插入查询结果

语法

INSERT INTO table_name [(column [, column ...])] SELECT ...

案例

删除表中的的重复复记录，重复的数据只能有一份

创建原数据表并插入数据

mysql> CREATE TABLE duplicate_table (id int, name varchar(20));
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO duplicate_table VALUES
    -> (100, 'aaa'),
    -> (100, 'aaa'),
    -> (200, 'bbb'),
    -> (200, 'bbb'),
    -> (200, 'bbb'),
    -> (300, 'ccc');
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> desc duplicate_table;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

创建一张空表 no_duplicate_table，结构和 duplicate_table 一样

mysql> create table no_duplicate_table like duplicate_table;
Query OK, 0 rows affected (0.02 sec)

mysql> desc no_duplicate_table;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

将 duplicate_table 的去重数据插入到 no_duplicate_table

mysql> insert into no_duplicate_table select distinct * from duplicate_table;
Query OK, 3 rows affected (0.03 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from no_duplicate_table;
+------+------+
| id   | name |
+------+------+
|  100 | aaa  |
|  200 | bbb  |
|  300 | ccc  |
+------+------+
3 rows in set (0.00 sec)

通过重命名表，实现原子的去重操作

mysql> rename table duplicate_table to old_duplicate_table;
Query OK, 0 rows affected (0.02 sec)

mysql> rename table no_duplicate_table to duplicate_table;
Query OK, 0 rows affected (0.01 sec)

查看最终结果

mysql> select * from duplicate_table;
+------+------+
| id   | name |
+------+------+
|  100 | aaa  |
|  200 | bbb  |
|  300 | ccc  |
+------+------+
3 rows in set (0.01 sec)

为什么最后是通过rename的方式进行的？

就是单纯的等一切都就绪了，然后统一放入、更新、生效等。确保操作是原子的。

聚合函数

函数介绍

函数	说明
COUNT([DISTINCT] expr)	返回查询到的数据的 数量
SUM([DISTINCT] expr)	返回查询到的数据的 总和，不是数字没有意义
AVG([DISTINCT] expr)	返回查询到的数据的 平均值，不是数字没有意义
MAX([DISTINCT] expr)	返回查询到的数据的 最大值，不是数字没有意义
MIN([DISTINCT] expr)	返回查询到的数据的 最小值，不是数字没有意义

案例

1. 统计班级共有多少同学

使用 * 做统计，不受 NULL 影响

mysql> select count(*) from exam_result;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in set (0.03 sec)

使用表达式做统计

mysql> select count(1) from exam_result;
+----------+
| count(1) |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)

2. 统计班级收集的 qq 号有多少

NULL 不会计入结果

mysql> select count(qq) from students;
+-----------+
| count(qq) |
+-----------+
|         9 |
+-----------+
1 row in set (0.00 sec)

3. 统计本次考试的数学成绩分数个数

COUNT(math) 统计的是全部成绩

mysql> select count(math) from exam_result;
+-------------+
| count(math) |
+-------------+
|           5 |
+-------------+
1 row in set (0.00 sec)

COUNT(DISTINCT math) 统计的是去重成绩数量

mysql> select count(distinct math) from exam_result;
+----------------------+
| count(distinct math) |
+----------------------+
|                    4 |
+----------------------+
1 row in set (0.02 sec)

4. 统计数学成绩总分

如果没有结果，则返回 NULL

mysql> select sum(math) from exam_result;
+-----------+
| sum(math) |
+-----------+
|       454 |
+-----------+
1 row in set (0.00 sec)

mysql> select sum(math) from exam_result where math<60;
+-----------+
| sum(math) |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)

5. 统计平均总分

mysql> select avg(chinese+math+english) 平均总分 from exam_result;
+--------------+
| 平均总分     |
+--------------+
|          303 |
+--------------+
1 row in set (0.00 sec)

6. 返回英语最高分

mysql> select max(english) from exam_result;
+--------------+
| max(english) |
+--------------+
|           90 |
+--------------+
1 row in set (0.01 sec)

7. 返回 > 70 分以上的英语最低分

mysql> select min(english) from exam_result where english>70;
+--------------+
| min(english) |
+--------------+
|           78 |
+--------------+
1 row in set (0.00 sec)

group by子句的使用

在select中使用group by 子句可以对指定列进行分组查询

语法

select column1, column2, .. from table group by column;

分组的目的是为了进行分组以后，方便进行聚合统计

having和where

where：对具体的任意列进行条件筛选
having：对分组聚合之后的结果进行条件筛选

案例

1. 准备工作，创建一个雇员信息表（来自oracle 9i的经典测试表）

• EMP员工表
• DEPT部门表
• SALGRADE工资等级表

具体代码可私我

2. 如何显示每个部门的平均工资和最高工资

mysql> select deptno,avg(sal),max(sal) from emp group by deptno;
+--------+-------------+----------+
| deptno | avg(sal)    | max(sal) |
+--------+-------------+----------+
|     10 | 2916.666667 |  5000.00 |
|     20 | 2175.000000 |  3000.00 |
|     30 | 1566.666667 |  2850.00 |
+--------+-------------+----------+
3 rows in set (0.00 sec)

3. 显示每个部门的每种岗位的平均工资和最低工资

mysql> select avg(sal),min(sal),job, deptno from emp group by deptno, job;
+-------------+----------+-----------+--------+
| avg(sal)    | min(sal) | job       | deptno |
+-------------+----------+-----------+--------+
| 1300.000000 |  1300.00 | CLERK     |     10 |
| 2450.000000 |  2450.00 | MANAGER   |     10 |
| 5000.000000 |  5000.00 | PRESIDENT |     10 |
| 3000.000000 |  3000.00 | ANALYST   |     20 |
|  950.000000 |   800.00 | CLERK     |     20 |
| 2975.000000 |  2975.00 | MANAGER   |     20 |
|  950.000000 |   950.00 | CLERK     |     30 |
| 2850.000000 |  2850.00 | MANAGER   |     30 |
| 1400.000000 |  1250.00 | SALESMAN  |     30 |
+-------------+----------+-----------+--------+
9 rows in set (0.00 sec)

4. 显示平均工资低于2000的部门和它的平均工资

统计各个部门的平均工资

mysql> select avg(sal) from emp group by deptno;
+-------------+
| avg(sal)    |
+-------------+
| 2916.666667 |
| 2175.000000 |
| 1566.666667 |
+-------------+
3 rows in set (0.00 sec)

having和group by配合使用，对group by结果进行过滤

mysql> select avg(sal) as myavg from emp group by deptno having myavg<2000;
+-------------+
| myavg       |
+-------------+
| 1566.666667 |
+-------------+
1 row in set (0.00 sec)

结语

不要单纯的认为，只有磁盘上的表结构导入到MySQL，真实存在的表才叫表。中间筛选出来的，包括最终结果，都可以看成逻辑上的表。
MySQL一切皆表

未来我们只要能够处理好单表的CURD，所有sql场景，我们全部都能用统一的方式进行。

⚠️ 写在最后：以上内容是我在学习以后得一些总结和概括，如有错误或者需要补充的地方欢迎各位大佬评论或者私信我交流！！！