SQL语句流程函数

本人因为今天用到了流程函数，顿时感觉语法生疏啊，为了防止以后忘记，故写此篇！！！

流程函数是MySQL相对常用的一类函数, 用户可以使用这类函数在一个SQL语句中实现条件选择, 这样能够提高效率.

下面列出了MySQL跟条件有关的流程函数

函数	功能
IF(expr1,expr2,expr3)	如果expr1是真, 返回expr2, 否则返回expr3
IFNULL(expr1,expr2)	如果expr1不是NULL,返回expr1,否则返回expr2
CASE WHEN [value1] THEN[result1]… ELSE[default] END	如果value是真, 返回result1,否则返回default
CASE [expr] WHEN [value1] THEN[result1]… ELSE[default] END	如果expr等于value1, 返回result1,否则返回default

下面的例子中模拟对职员进行分类,首先创建一个职员薪水表:

1	create table salary(userid int, salary decimal(9,2));

插入一些测试数据

1	insert into salary values (1,1000),(2,2000),(3,3000),(4,4000),(5,5000),(1,null);

数据如下

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mysql> select * from salary; +--------+---------+ | userid | salary | +--------+---------+ | 1 | 1000.00 | | 2 | 2000.00 | | 3 | 3000.00 | | 4 | 4000.00 | | 5 | 5000.00 | | 1 | NULL | +--------+---------+ 6 rows in set (0.00 sec)

接下来, 通过这个表来介绍各个函数的应用.

IF(expr1,expr2,expr3)函数: 这里认为月薪在2000元以上的职员属于高薪, 用"high'表示; 而2000以下的职员属于低薪, 用'low'来表示.

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mysql> select if(salary>2000, 'high', 'low') from salary;        +--------------------------------+ | if(salary>2000, 'high', 'low') | +--------------------------------+ | low                            | | low                            | | high                           | | high                           | | high                           | | low                            | +--------------------------------+ 6 rows in set (0.00 sec)

IFNULL(expr1,expr2)函数: 这个函数一般用来替换NULL值, 我们知道NULL值是不能参参与数值运算的, 下面这个语句就是把NULL值用0替换.

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mysql> select ifnull(salary,0) from salary; +------------------+ | ifnull(salary,0) | +------------------+ |          1000.00 | |          2000.00 | |          3000.00 | |          4000.00 | |          5000.00 | |             0.00 | +------------------+ 6 rows in set (0.00 sec)

CASE WHEN [value1] THEN[result1]… ELSE[default] END函数:这里可以用case when..then函数实现上面例子中高薪低薪的问题.

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mysql> select CASE WHEN salary<=2000 THEN 'low' else 'high' END  from salary; +---------------------------------------------------+ | CASE WHEN salary<=2000 THEN 'low' else 'high' END | +---------------------------------------------------+ | low                                               | | low                                               | | high                                              | | high                                              | | high                                              | | high                                              | +---------------------------------------------------+ 6 rows in set (0.00 sec)

CASE [expr] WHEN [value1] THEN[result1]… ELSE[default] END函数：这里还可以分为多种情况把职员的薪水分为多个档次，比如下面的例子分成高、中、低３种情况。同样可以分成更多种情况，这里不再举例了，有兴趣的朋友可以自己测试一下

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mysql> select CASE salary WHEN 1000 THEN 'low' when 2000 THEN 'mid' ELSE 'high' END from salary; +-----------------------------------------------------------------------+ | CASE salary WHEN 1000 THEN 'low' when 2000 THEN 'mid' ELSE 'high' END | +-----------------------------------------------------------------------+ | low                                                                   | | mid                                                                   | | high                                                                  | | high                                                                  | | high                                                                  | | high                                                                  | +-----------------------------------------------------------------------+ 6 rows in set (0.00 sec)