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case when then else end
语句用于查询满足多种条件的情况，类似java中的if...else，还有的就是用于进行行转列的查询，这个是放在select 子句后面的，是充当的是字段的作用。
具体用法就是：分为两种，一种是简单的函数形式，另一种就是表达式的形式。
其含义解释：
简单的函数形式
case 字段 when 值 then 结果 else 其他情况 end；
表达式的形式
case when 字段=值(这里写表达式，例如 score=80) then 结果 else 其他情况 end；
简单的函数形式
select case score when 'a' then '优秀' else '良好' end from student;
表达式形式
select case when score between and 90 then '优秀' else '良好' end from student;
在case里面还可以嵌套子查询等复杂用法。
case when 可以把它当成一个字段，其实就是select 后面需要查询出来的一个字段，对其可以使用聚合函数，别名，去重，等操作。
if的用法 Mysql的if既可以作为表达式用，也可在存储过程中作为流程控制语句使用，如下是做为表达式使用：
三目表达式:
IF(expr1,expr2,expr3) 如果 expr1 是TRUE (expr1 <> 0 and expr1 <> NULL)，则
IF()的返回值为expr2; 否则返回值则为 expr3。IF() 的返回值为数字值或字符串值，具体情况视其所在语境而定。
if else的用法 if实现条件判断，满足不同条件执行不同的操作，做为流程控制语句使用。
IF search_condition THEN statement_list ELSE statement_list END IF;
Mysql的if既可以作为表达式用，也可在存储过程中作为流程控制语句使用，如下是做为表达式使用：
select DISTINCT if (EXISTS( select user_name FROM user WHERE
user_name='12344123'),1,0) a from user
