本文以实例形式来讲解MySQL的函数
按照功能的分类
1 字符函数
2 数值运算符与函数
3 比较运算符与函数
4 日期时间函数
5 信息函数
6 聚合函数
7 加密函数
字符函数分类
CONCAT() 字符连接
CONCAT_WS() 使用指定的分隔符进行连接
FORMAT() 数字格式化
LOWER() 转换成小写字母
UPPER() 转换成大写字母
LEFT() 获取左侧字符
RIGHT() 获取右侧字符
字符函数分类
CONCAT() 字符连接
CONCAT_WS() 使用指定的分隔符进行连接
FORMAT() 数字格式化
LOWER() 转换成小写字母
UPPER() 转换成大写字母
LEFT() 获取左侧字符
RIGHT() 获取右侧字符
CONCAT()字符连接，数据表的名字后面可以加东西
例1
mysql> SELECT CONCAT('provinces','--');
+--------------------------+
| CONCAT('provinces','--') |
+--------------------------+
| provinces-- |
+--------------------------+
1 row in set (0.00 sec)
创建一张演示表
mysql> CREATE TABLE testwish(
-> first_name VARCHAR(20) DEFAULT NULL,
-> last_name VARCHAR(10) DEFAULT NULL
-> );
Query OK, 0 rows affected (0.01 sec)
mysql> SHOW COLUMNS FROM testwish;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| first_name | varchar(20) | YES | | NULL | |
| last_name | varchar(10) | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)
插入一些记录
mysql> INSERT testwish(first_name,last_name) VALUES('QQ','121'),('AC','BD'),('121','725');
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM testwish;
+------------+-----------+
| first_name | last_name |
+------------+-----------+
| QQ | 121 |
| AC | BD |
| 121 | 725 |
+------------+-----------+
3 rows in set (0.00 sec)
例2
把这两个字段合并在一起输出，用AS起个别名
mysql> SELECT CONCAT(first_name,last_name) AS fullname FROM testwish;
+----------+
| fullname |
+----------+
| QQ121 |
| ACBD |
| 121725 |
+----------+
3 rows in set (0.00 sec)
CONCAT_WS()需要至少三个参数
例3，字符连接
mysql> SELECT CONCAT_WS('|','A','B','C');
+----------------------------+
| CONCAT_WS('|','A','B','C') |
+----------------------------+
| A|B|C |
+----------------------------+
1 row in set (0.00 sec)
mysql>
mysql> SELECT CONCAT_WS('-','test1','test2');
+--------------------------------+
| CONCAT_WS('-','test1','test2') |
+--------------------------------+
| test1-test2 |
+--------------------------------+
1 row in set (0.00 sec)
mysql> SELECT CONCAT_WS('-','test1','test2','test');
+---------------------------------------+
| CONCAT_WS('-','test1','test2','test') |
+---------------------------------------+
| test1-test2-test |
+---------------------------------------+
1 row in set (0.00 sec)
例4
截取小数点后两位数字
mysql> SELECT FORMAT(12580.45632,2);
+-----------------------+
| FORMAT(12580.45632,2) |
+-----------------------+
| 12,580.46 |
+-----------------------+
1 row in set (0.00 sec)
例5
大写转换成小写
mysql> SELECT LOWER('MySQL');
+----------------+
| LOWER('MySQL') |
+----------------+
| mysql |
+----------------+
1 row in set (0.00 sec)
例6
小写转换成大写
mysql> SELECT UPPER('mysql');
+----------------+
| UPPER('mysql') |
+----------------+
| MYSQL |
+----------------+
1 row in set (0.00 sec)
例7
获取左侧N个字符
mysql> SELECT LEFT('mysql',2);
+-----------------+
| LEFT('mysql',2) |
+-----------------+
| my |
+-----------------+
1 row in set (0.00 sec)
例8 获取左侧N个字符，并转换为小写。用嵌套
mysql> SELECT LOWER(LEFT('MySQL',2));
+------------------------+
| LOWER(LEFT('MySQL',2)) |
+------------------------+
| my |
+------------------------+
1 row in set (0.00 sec)
例9
获取右侧 N个字符
mysql> SELECT RIGHT('MySQL',2);
+------------------+
| RIGHT('MySQL',2) |
+------------------+
| QL |
+------------------+
1 row in set (0.00 sec)
字符函数
LENGTH() 获取字符串长度
LTRIM() 删除前导空格
RTRIM() 删除后续空格
TRIM() 删除前导和后续空格
SUBSTRING() 字符串截取
[NOT]LIKE 模式匹配
PERLACE() 字符串替换
例10 获取字符串长度
mysql> SELECT LENGTH('MySQL');
+-----------------+
| LENGTH('MySQL') |
+-----------------+
| 5 |
+-----------------+
1 row in set (0.00 sec)
注意如果有空格，也会算在里面
mysql> SELECT LENGTH(' My SQL');
+---------------------+
| LENGTH(' My SQL') |
+---------------------+
| 9 |
+---------------------+
1 row in set (0.00 sec)
前导空格，是第一个字符之前 的空格，后续空格，是最后一个字符之后的空格
例11，删除前导空格，并查看长度
mysql> SELECT LENGTH (' MySQL ');
+------------------------+
| LENGTH (' MySQL ') |
+------------------------+
| 11 |
+------------------------+
1 row in set (0.00 sec)
mysql> SELECT LENGTH (LTRIM(' MySQL '));
+-------------------------------+
| LENGTH (LTRIM(' MySQL ')) |
+-------------------------------+
| 9 |
+-------------------------------+
1 row in set (0.00 sec)
例12，删除 后续空格，并查看长度
mysql> SELECT LENGTH (RTRIM(' MySQL '));
+-------------------------------+
| LENGTH (RTRIM(' MySQL ')) |
+-------------------------------+
| 7 |
+-------------------------------+
1 row in set (0.00 sec)
例13 去掉所有空格，并查看长度
mysql> SELECT LENGTH (TRIM(' MySQL '));
+------------------------------+
| LENGTH (TRIM(' MySQL ')) |
+------------------------------+
| 5 |
+------------------------------+
1 row in set (0.00 sec)
例14 删除前导中的“?”
mysql> SELECT TRIM(LEADING '?' FROM '??MySQL???');
+-------------------------------------+
| TRIM(LEADING '?' FROM '??MySQL???') |
+-------------------------------------+
| MySQL??? |
+-------------------------------------+
1 row in set (0.00 sec)
例15 删除后续中的”?”
mysql> SELECT TRIM(TRAILING '?' FROM '??MySQL???');
+--------------------------------------+
| TRIM(TRAILING '?' FROM '??MySQL???') |
+--------------------------------------+
| ??MySQL |
+--------------------------------------+
1 row in set (0.00 sec)
例16 删除所有的”?”
mysql> SELECT TRIM(BOTH '?' FROM '??MySQL???');
+----------------------------------+
| TRIM(BOTH '?' FROM '??MySQL???') |
+----------------------------------+
| MySQL |
+----------------------------------+
1 row in set (0.00 sec)
注意BOTH 删除不掉中间的”?”
mysql> SELECT TRIM(BOTH '?' FROM '??My??SQL???');
+------------------------------------+
| TRIM(BOTH '?' FROM '??My??SQL???') |
+------------------------------------+
| My??SQL |
+------------------------------------+
1 row in set (0.00 sec)
例17用字符串替换掉字符串
mysql> SELECT REPLACE('??My??SQL????','?','--');
+-----------------------------------+
| REPLACE('??My??SQL????','?','--') |
+-----------------------------------+
| ----My----SQL-------- |
+-----------------------------------+
1 row in set (0.00 sec)
mysql> SELECT REPLACE('??My??SQL????','?','');
+---------------------------------+
| REPLACE('??My??SQL????','?','') |
+---------------------------------+
| MySQL |
+---------------------------------+
1 row in set (0.00 sec)
例18 用字符串替换掉字符串,一对多
mysql> SELECT REPLACE('??My??SQL????','?','-+-!!');
+-----------------------------------------------+
| REPLACE('??My??SQL????','?','-+-!!') |
+-----------------------------------------------+
| -+-!!-+-!!My-+-!!-+-!!SQL-+-!!-+-!!-+-!!-+-!! |
+-----------------------------------------------+
1 row in set (0.00 sec)
例19用字符串替换掉字符串，多对一
mysql> SELECT REPLACE('??My??SQL????','??','~');
+-----------------------------------+
| REPLACE('??My??SQL????','??','~') |
+-----------------------------------+
| ~My~SQL~~ |
+-----------------------------------+
1 row in set (0.00 sec)
例20，字符串截取，从第一位开始，截取两个
mysql> SELECT SUBSTRING('MySQL',1,2);
+------------------------+
| SUBSTRING('MySQL',1,2) |
+------------------------+
| My |
+------------------------+
1 row in set (0.00 sec)
注意不指定位置的情况
mysql> SELECT SUBSTRING('MySQL',3);
+----------------------+
| SUBSTRING('MySQL',3) |
+----------------------+
| SQL |
+----------------------+
1 row in set (0.00 sec)
例21 倒序截取
mysql> SELECT SUBSTRING('MySQL',-2);
+-----------------------+
| SUBSTRING('MySQL',-2) |
+-----------------------+
| QL |
+-----------------------+
1 row in set (0.00 sec)
模式的匹配,%表示0个或多个字符，‘1’表示TRUE
mysql> SELECT 'MySQL' LIKE 'M%';
+-------------------+
| 'MySQL' LIKE 'M%' |
+-------------------+
| 1 |
+-------------------+
1 row in set (0.00 sec)
例22 查找第一个字符组中包含‘Q’的用户
mysql> SELECT * FROM testwish WHERE first_name LIKE '%Q%';
+------------+-----------+
| first_name | last_name |
+------------+-----------+
| QQ | 121 |
+------------+-----------+
1 row in set (0.00 sec)
当前表
mysql> SELECT * FROM testwish;
+------------+-----------+
| first_name | last_name |
+------------+-----------+
| QQ | 121 |
| AC | BD |
| 121 | 725 |
| tom% | 2016 |
+------------+-----------+
例子23查找 包含% 的用户
告诉字符串，1 后面的不需要再进行通配符解析，直接默认为是标准的%
mysql> SELECT * FROM testwish WHERE first_name LIKE '%1%%' ESCAPE '1';
+------------+-----------+
| first_name | last_name |
+------------+-----------+
| tom% | 2016 |
+------------+-----------+
