MySQL内置函数
文章目录
- 日期函数
- 字符串函数
- 数学函数
- 其他函数
日期函数
获取年月日：
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2023-02-01 |
+----------------+
1 row in set (0.00 sec)
获得时分秒：
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 01:10:26 |
+----------------+
1 row in set (0.00 sec)
获取时间戳：
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2023-02-01 01:11:08 |
+---------------------+
1 row in set (0.00 sec)
在日期的基础上加日期：
mysql> select date_add('2013-10-01', interval 10 day);
+-----------------------------------------+
| date_add('2013-10-01', interval 10 day) |
+-----------------------------------------+
| 2013-10-11 |
+-----------------------------------------+
1 row in set (0.00 sec)
在日期的基础上减去时间：
mysql> select date_sub('2013-10-01', interval 10 day);
+-----------------------------------------+
| date_sub('2013-10-01', interval 10 day) |
+-----------------------------------------+
| 2013-09-21 |
+-----------------------------------------+
1 row in set (0.00 sec)
计算两个日期之间相差多少天：
mysql> select datediff('2023-02-01', '1949-10-01');
+--------------------------------------+
| datediff('2023-02-01', '1949-10-01') |
+--------------------------------------+
| 26786 |
+--------------------------------------+
1 row in set (0.00 sec)
案例：
创建一张表，记录生日：
mysql> create table birthday (
-> id int unsigned primary key auto_increment,
-> birthday date
-> );
Query OK, 0 rows affected (0.02 sec)
添加当前日期：
mysql> insert into birthday (birthday) values (current_date());
Query OK, 1 row affected (0.00 sec)
mysql> select * from birthday;
+----+------------+
| id | birthday |
+----+------------+
| 1 | 2023-02-01 |
+----+------------+
1 row in set (0.00 sec)
案例：
创建一个留言表：
mysql> create table msg (
-> id int unsigned primary key auto_increment,
-> content varchar(30) not null,
-> sendtime datetime
-> );
Query OK, 0 rows affected (0.02 sec)
插入数据：
mysql> insert into msg (content, sendtime) values ('hello world', now());
Query OK, 1 row affected (0.01 sec)
mysql> insert into msg (content, sendtime) values ('hello mysql', now());
Query OK, 1 row affected (0.01 sec)
mysql> select * from msg;
+----+-------------+---------------------+
| id | content | sendtime |
+----+-------------+---------------------+
| 1 | hello world | 2023-02-01 01:21:23 |
| 2 | hello mysql | 2023-02-01 01:21:30 |
+----+-------------+---------------------+
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> select content, date(sendtime) from msg;
+-------------+----------------+
| content | date(sendtime) |
+-------------+----------------+
| hello world | 2023-02-01 |
| hello mysql | 2023-02-01 |
+-------------+----------------+
2 rows in set (0.00 sec)
查询在2分钟内发布的帖子：
mysql> select * from msg;
+----+-------------+---------------------+
| id | content | sendtime |
+----+-------------+---------------------+
| 1 | hello world | 2023-02-01 01:21:23 |
| 2 | hello mysql | 2023-02-01 01:21:30 |
| 3 | hello Linux | 2023-02-01 01:24:34 |
+----+-------------+---------------------+
3 rows in set (0.00 sec)
mysql> select content, sendtime from msg where date_add(sendtime, interval 2 minute) > now();
+-------------+---------------------+
| content | sendtime |
+-------------+---------------------+
| hello Linux | 2023-02-01 01:24:34 |
+-------------+---------------------+
1 row in set (0.00 sec)
字符串函数
获取 emp表的 ename列的字符集：
mysql> select charset(ename) from emp limit 1;
+----------------+
| charset(ename) |
+----------------+
| utf8 |
+----------------+
1 row in set (0.00 sec)
要求显示exam_result表中的信息，显示格式： “XXX的语文是XXX分，数学XXX分，英语XXX分”：
mysql> select concat(name, '的语文是', chinese, '分，数学', math, '分，英语', english, '分') from exam_result;
+---------------------------------------------------------------------------------------------+
| concat(name, '的语文是', chinese, '分，数学', math, '分，英语', english, '分') |
+---------------------------------------------------------------------------------------------+
| 唐三藏的语文是134分，数学98分，英语56分 |
| 猪悟能的语文是176分，数学98分，英语90分 |
| 曹孟德的语文是140分，数学90分，英语67分 |
| 刘玄德的语文是110分，数学115分，英语45分 |
| 孙权的语文是140分，数学73分，英语78分 |
| 宋公明的语文是150分，数学95分，英语30分 |
+---------------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)
求学生表中学生姓名占用的字节数
mysql> select name, length(name) from students;
+----------+--------------+
| name | length(name) |
+----------+--------------+
| Curry | 5 |
| Durant | 6 |
| Bryant | 6 |
| Mitchell | 8 |
+----------+--------------+
4 rows in set (0.00 sec)
将EMP表中所有名字中有S的替换成’数据库’
mysql> select replace(ename, 'S', '数据库'), ename from emp;
+----------------------------------+--------+
| replace(ename, 'S', '数据库') | ename |
+----------------------------------+--------+
| 数据库MITH | SMITH |
| ALLEN | ALLEN |
| WARD | WARD |
| JONE数据库 | JONES |
| MARTIN | MARTIN |
| BLAKE | BLAKE |
| CLARK | CLARK |
| 数据库COTT | SCOTT |
| KING | KING |
| TURNER | TURNER |
| ADAM数据库 | ADAMS |
| JAME数据库 | JAMES |
| FORD | FORD |
| MILLER | MILLER |
+----------------------------------+--------+
14 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> select substring(ename, 2, 2), ename from emp;
+------------------------+--------+
| substring(ename, 2, 2) | ename |
+------------------------+--------+
| MI | SMITH |
| LL | ALLEN |
| AR | WARD |
| ON | JONES |
| AR | MARTIN |
| LA | BLAKE |
| LA | CLARK |
| CO | SCOTT |
| IN | KING |
| UR | TURNER |
| DA | ADAMS |
| AM | JAMES |
| OR | FORD |
| IL | MILLER |
+------------------------+--------+
14 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> select concat(lcase(substring(ename, 1, 1)), substring(ename, 2)) from emp;
+------------------------------------------------------------+
| concat(lcase(substring(ename, 1, 1)), substring(ename, 2)) |
+------------------------------------------------------------+
| sMITH |
| aLLEN |
| wARD |
| jONES |
| mARTIN |
| bLAKE |
| cLARK |
| sCOTT |
| kING |
| tURNER |
| aDAMS |
| jAMES |
| fORD |
| mILLER |
+------------------------------------------------------------+
14 rows in set (0.00 sec)
数学函数
绝对值：
mysql> select abs(-200);
+-----------+
| abs(-200) |
+-----------+
| 200 |
+-----------+
1 row in set (0.00 sec)
向上取整：
mysql> select ceiling(8.1);
+--------------+
| ceiling(8.1) |
+--------------+
| 9 |
+--------------+
1 row in set (0.00 sec)
mysql> select ceiling(-8.1);
+---------------+
| ceiling(-8.1) |
+---------------+
| -8 |
+---------------+
1 row in set (0.00 sec)
向下取整：
mysql> select floor(8.6);
+------------+
| floor(8.6) |
+------------+
| 8 |
+------------+
1 row in set (0.00 sec)
mysql> select floor(-8.6);
+-------------+
| floor(-8.6) |
+-------------+
| -9 |
+-------------+
1 row in set (0.00 sec)
保留2位小数位数（小数四舍五入)：
mysql> select format(12.456, 1);
+-------------------+
| format(12.456, 1) |
+-------------------+
| 12.5 |
+-------------------+
1 row in set (0.00 sec)
十进制转二进制：
mysql> select bin(12);
+---------+
| bin(12) |
+---------+
| 1100 |
+---------+
1 row in set (0.00 sec)
转化成十六进制：
mysql> select hex(10);
+---------+
| hex(10) |
+---------+
| A |
+---------+
1 row in set (0.00 sec)
随机数：
mysql> select rand();
+--------------------+
| rand() |
+--------------------+
| 0.7938835246431488 |
+--------------------+
1 row in set (0.00 sec)
其他函数
user() 查询当前用户：
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，相当于对内容进行加密：
mysql> select md5('root');
+----------------------------------+
| md5('root') |
+----------------------------------+
| 63a9f0ea7bb98050796b649e85481845 |
+----------------------------------+
1 row in set (0.00 sec)
database()显示当前正在使用的数据库：
mysql> select database();
+------------+
| database() |
+------------+
| scott |
+------------+
1 row in set (0.00 sec)
ifnull（val1， val2） 如果val1为null，返回val2，否则返回val1的值：
mysql> select ifnull('a', 123);
+------------------+
| ifnull('a', 123) |
+------------------+
| a |
+------------------+
1 row in set (0.00 sec)
mysql> select ifnull(null, 123);
+-------------------+
| ifnull(null, 123) |
+-------------------+
| 123 |
+-------------------+
1 row in set (0.00 sec)