文章目录
- [7. 函数](#7. 函数)
-
- [7.1 日期函数](#7.1 日期函数)
-
- [7.1.1 案例1](#7.1.1 案例1)
- [7.1.2 案例2](#7.1.2 案例2)
- [7.2 字符串函数](#7.2 字符串函数)
- [7.3 数学函数](#7.3 数学函数)
- [7.4 其它函数](#7.4 其它函数)
7. 函数
7.1 日期函数
- 获得年月日：
mysql
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2025-05-06 |
+----------------+
1 row in set (0.00 sec)- 获得时分秒：
mysql
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 21:38:38 |
+----------------+
1 row in set (0.00 sec)- 获得时间戳：
mysql
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2025-05-06 21:38:48 |
+---------------------+
1 row in set (0.00 sec)- 在日期的基础上加日期：
mysql
mysql> select date_add('2017-10-28', interval 10 day);
+-----------------------------------------+
| date_add('2017-10-28', interval 10 day) |
+-----------------------------------------+
| 2017-11-07 |
+-----------------------------------------+
1 row in set (0.00 sec)- 在日期的基础上减去时间：
mysql
mysql> select date_sub('2017-10-1', interval 2 day);
+---------------------------------------+
| date_sub('2017-10-1', interval 2 day) |
+---------------------------------------+
| 2017-09-29 |
+---------------------------------------+
1 row in set (0.00 sec)- 计算两个日期之间相差多少天：
mysql
mysql> select datediff('2017-10-10', '2016-9-1');
+------------------------------------+
| datediff('2017-10-10', '2016-9-1') |
+------------------------------------+
| 404 |
+------------------------------------+
1 row in set (0.00 sec)7.1.1 案例1
创建一张表，记录生日
mysql
mysql> create table tmp(
-> id int primary key auto_increment,
-> birthday date
-> );
Query OK, 0 rows affected (0.02 sec)添加当前日期：
mysql
mysql> insert into tmp(birthday) values(current_date());
Query OK, 1 row affected (0.01 sec)
mysql> insert into tmp(birthday) values(current_date());
Query OK, 1 row affected (0.01 sec)
mysql> select * from tmp;
+----+------------+
| id | birthday |
+----+------------+
| 1 | 2025-05-06 |
| 2 | 2025-05-06 |
+----+------------+
2 rows in set (0.00 sec)7.1.2 案例2
创建一个留言表
mysql
mysql> create table msg (
-> id int primary key auto_increment,
-> content varchar(30) not null,
-> sendtime datetime
-> );
Query OK, 0 rows affected (0.02 sec)插入数据
mysql
mysql> insert into msg(content,sendtime) values('hello1', now());
Query OK, 1 row affected (0.00 sec)
mysql> insert into msg(content,sendtime) values('hello2', now());
Query OK, 1 row affected (0.00 sec)
mysql> select * from msg;
+----+---------+---------------------+
| id | content | sendtime |
+----+---------+---------------------+
| 1 | hello1 | 2025-05-06 21:42:33 |
| 2 | hello2 | 2025-05-06 21:42:38 |
+----+---------+---------------------+
2 rows in set (0.00 sec)显示所有留言信息，发布日期只显示日期，不用显示时间
mysql
mysql> select content,date(sendtime) from msg;
+---------+----------------+
| content | date(sendtime) |
+---------+----------------+
| hello1 | 2025-05-06 |
| hello2 | 2025-05-06 |
+---------+----------------+
2 rows in set (0.00 sec)请查询在2分钟内发布的帖子
mysql
mysql> select * from msg where date_add(sendtime, interval 2 minute) > now();
+----+---------+---------------------+
| id | content | sendtime |
+----+---------+---------------------+
| 1 | hello1 | 2025-05-06 21:42:33 |
| 2 | hello2 | 2025-05-06 21:42:38 |
+----+---------+---------------------+
2 rows in set (0.00 sec)
理解：
------------------------------|-----------|-------------|------------------
初始时间 now() 初始时间+2min 7.2 字符串函数
案例：
设计emp表
mysql
mysql> CREATE TABLE `emp` (
-> `empno` int(6) unsigned zerofill NOT NULL COMMENT '雇员编号',
-> `ename` varchar(20) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL COMMENT '雇员姓名',
-> `job` varchar(15) DEFAULT NULL COMMENT '雇员职位',
-> `mgr` int(4) unsigned zerofill DEFAULT NULL COMMENT '雇员领导编号',
-> `hiredate` datetime DEFAULT NULL COMMENT '雇佣时间',
-> `sal` decimal(7,2) DEFAULT NULL COMMENT '工资月薪',
-> `comm` decimal(7,2) DEFAULT NULL COMMENT '奖金',
-> `deptno` int(2) unsigned zerofill DEFAULT NULL COMMENT '部门编号',
-> PRIMARY KEY (`empno`),
-> KEY `idx_ename` (`ename`),
-> KEY `idx_job` (`job`),
-> KEY `idx_deptno` (`deptno`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='员工信息表';
Query OK, 0 rows affected, 9 warnings (0.03 sec)插入数据
mysql
mysql> INSERT INTO emp VALUES
-> (007369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800.00, NULL, 20),
-> (007499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600.00, 300.00, 30),
-> (007521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250.00, 500.00, 30),
-> (007566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975.00, NULL, 20),
-> (007654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250.00, 1400.00, 30),
-> (007698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850.00, NULL, 30),
-> (007782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450.00, NULL, 10),
-> (007788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000.00, NULL, 20),
-> (007839, 'KING', 'PRESIDENT', NULL, '1981-11-17', 5000.00, NULL, 10),
-> (007844, 'TURNER', 'SALESMAN', 7698, '1981-09-08', 1500.00, 0.00, 30),
-> (007876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100.00, NULL, 20),
-> (007900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950.00, NULL, 30),
-> (007902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000.00, NULL, 20),
-> (007934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300.00, NULL, 10),
-> (008001, '张三', '程序员', 7566, '2023-01-15', 4500.00, NULL, 20),
-> (008002, '李四', '销售代表', 7698, '2023-02-01', 3800.00, 600.00, 30);
Query OK, 16 rows affected (0.00 sec)
Records: 16 Duplicates: 0 Warnings: 0获取emp表的ename列的字符集
mysql
mysql> SELECT charset(ename) FROM emp;
+----------------+
| charset(ename) |
+----------------+
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
| utf8mb3 |
+----------------+
16 rows in set (0.00 sec)要求显示exam_result表中的信息，显示格式："XXX的工资是XXX元，职位是XXX元"
mysql
mysql> SELECT CONCAT(ename, '的工资是', sal, '元，职位是', job) AS '员工信息'
-> FROM emp;
+------------------------------------------------------+
| 员工信息 |
+------------------------------------------------------+
| SMITH的工资是800.00元，职位是CLERK |
| ALLEN的工资是1600.00元，职位是SALESMAN |
| WARD的工资是1250.00元，职位是SALESMAN |
| JONES的工资是2975.00元，职位是MANAGER |
| MARTIN的工资是1250.00元，职位是SALESMAN |
| BLAKE的工资是2850.00元，职位是MANAGER |
| CLARK的工资是2450.00元，职位是MANAGER |
| SCOTT的工资是3000.00元，职位是ANALYST |
| KING的工资是5000.00元，职位是PRESIDENT |
| TURNER的工资是1500.00元，职位是SALESMAN |
| ADAMS的工资是1100.00元，职位是CLERK |
| JAMES的工资是950.00元，职位是CLERK |
| FORD的工资是3000.00元，职位是ANALYST |
| MILLER的工资是1300.00元，职位是CLERK |
| 张三的工资是4500.00元，职位是程序员 |
| 李四的工资是3800.00元，职位是销售代表 |
+------------------------------------------------------+
16 rows in set (0.00 sec)求学生表中学生姓名占用的字节数
mysql
mysql> SELECT LENGTH(ename) AS name_bytes, ename
-> FROM emp;
+------------+--------+
| name_bytes | ename |
+------------+--------+
| 5 | ADAMS |
| 5 | ALLEN |
| 5 | BLAKE |
| 5 | CLARK |
| 4 | FORD |
| 5 | JAMES |
| 5 | JONES |
| 4 | KING |
| 6 | MARTIN |
| 6 | MILLER |
| 5 | SCOTT |
| 5 | SMITH |
| 6 | TURNER |
| 4 | WARD |
| 6 | 张三 |
| 6 | 李四 |
+------------+--------+
16 rows in set (0.00 sec)注意：
length函数返回字符串长度，以字节为单位。如果是多字节字符则计算多个字节数,如果是单字节字符则算作一个字节。
比如：字母，数字算作一个字节，中文表示多个字节数（与字符集编码有关）
将EMP表中所有名字中有S的替换成'上海'
mysql
mysql> SELECT REPLACE(ename, 'S', '上海'), ename
-> FROM emp;
+-------------------------------+--------+
| REPLACE(ename, 'S', '上海') | ename |
+-------------------------------+--------+
| ADAM上海 | ADAMS |
| ALLEN | ALLEN |
| BLAKE | BLAKE |
| CLARK | CLARK |
| FORD | FORD |
| JAME上海 | JAMES |
| JONE上海 | JONES |
| KING | KING |
| MARTIN | MARTIN |
| MILLER | MILLER |
| 上海COTT | SCOTT |
| 上海MITH | SMITH |
| TURNER | TURNER |
| WARD | WARD |
| 张三 | 张三 |
| 李四 | 李四 |
+-------------------------------+--------+
16 rows in set (0.00 sec)截取EMP表中ename字段的第二个到第三个字符
mysql
mysql> SELECT SUBSTRING(ename, 2, 2) AS name_substr, ename
-> FROM emp;
+-------------+--------+
| name_substr | ename |
+-------------+--------+
| DA | ADAMS |
| LL | ALLEN |
| LA | BLAKE |
| LA | CLARK |
| OR | FORD |
| AM | JAMES |
| ON | JONES |
| IN | KING |
| AR | MARTIN |
| IL | MILLER |
| CO | SCOTT |
| MI | SMITH |
| UR | TURNER |
| AR | WARD |
| 三 | 张三 |
| 四 | 李四 |
+-------------+--------+
16 rows in set (0.00 sec)以首字母小写的方式显示所有员工的姓名
mysql
mysql> SELECT CONCAT(LCASE(SUBSTRING(ename, 1, 1)), SUBSTRING(ename, 2)) AS formatted_name, ename
-> FROM emp;
+----------------+--------+
| formatted_name | ename |
+----------------+--------+
| aDAMS | ADAMS |
| aLLEN | ALLEN |
| bLAKE | BLAKE |
| cLARK | CLARK |
| fORD | FORD |
| jAMES | JAMES |
| jONES | JONES |
| kING | KING |
| mARTIN | MARTIN |
| mILLER | MILLER |
| sCOTT | SCOTT |
| sMITH | SMITH |
| tURNER | TURNER |
| wARD | WARD |
| 张三 | 张三 |
| 李四 | 李四 |
+----------------+--------+
16 rows in set (0.00 sec)7.3 数学函数
绝对值
mysql
mysql> select abs(-100.2);
+-------------+
| abs(-100.2) |
+-------------+
| 100.2 |
+-------------+
1 row in set (0.00 sec)向上取整
mysql
mysql> select ceiling(23.04);
+----------------+
| ceiling(23.04) |
+----------------+
| 24 |
+----------------+
1 row in set (0.00 sec)向下取整
mysql
mysql> select floor(23.7);
+-------------+
| floor(23.7) |
+-------------+
| 23 |
+-------------+
1 row in set (0.00 sec)保留2位小数位数（小数四舍五入)
mysql
mysql> select format(12.3456, 2);
+--------------------+
| format(12.3456, 2) |
+--------------------+
| 12.35 |
+--------------------+
1 row in set (0.00 sec)产生随机数
mysql
mysql> select rand();
+--------------------+
| rand() |
+--------------------+
| 0.3962223784962877 |
+--------------------+
1 row in set (0.00 sec)7.4 其它函数
user() 查询当前用户
mysql
mysql> select user();
+----------------+
| user() |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)md5(str)对一个字符串进行md5摘要，摘要后得到一个32位字符串
mysql
mysql> select md5('admin');
+----------------------------------+
| md5('admin') |
+----------------------------------+
| 21232f297a57a5a743894a0e4a801fc3 |
+----------------------------------+
1 row in set (0.00 sec)database()显示当前正在使用的数据库
mysql
mysql> select database();
+------------+
| database() |
+------------+
| test_db |
+------------+
1 row in set (0.00 sec)password()函数，MySQL数据库使用该函数对用户加密
mysql
# 低版本
select password('root');
+-------------------------------------------+
| password('root') |
+-------------------------------------------+
| *81F5E21E35407D884A6CD4A631AEBFB6AF209E1B |
+-------------------------------------------+
# 高版本已弃用
mysql> select password('root');
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 '('root')' at line 1
ifnull（val1， val2）如果val1为null，返回val2，否则返回val1的值。如果
val1和val2都是NULL，IFNULL函数会返回NULL。
mysql
mysql> select ifnull('abc', '123');
+----------------------+
| ifnull('abc', '123') |
+----------------------+
| abc |
+----------------------+
1 row in set (0.00 sec)
mysql> select ifnull(null, '123');
+---------------------+
| ifnull(null, '123') |
+---------------------+
| 123 |
+---------------------+
1 row in set (0.00 sec)