mysql中的条件语句case when/if函数

主要知识点为case函数，if函数，ifnull函数，elt函数几部分，主要用于mysql语句中的逻辑判断

待操作的表如下：

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+----+-----------+-----+-------+--------+

+----+-----------+-----+-------+--------+

| 1 | xiaohong | 1 | 1 | 50 |

| 2 | xiaoming | 0 | 0 | 90 |

| 3 | xiaohuang | 1 | 2 | 80 |

| 4 | xiaoming | 0 | 3 | NULL |

+----+-----------+-----+-------+--------+

sex：0-woman，1-man，其中-unknown；level是客户的级别：1-normal，2-vip，3-vvip

1.case函数

语法1：

CASE expression

WHEN value1 THEN returnvalue1

WHEN value2 THEN returnvalue2

WHEN value3 THEN returnvalue3

……

ELSE defaultreturnvalue

END

举栗：

select t.name,(case t.sex when 0 then 'woman' when 1 then 'man' else 'unknown' end) sex from test t;

语法2：

CASE

WHEN condition1 THEN returnvalue1

WHEN condition 2 THEN returnvalue2

WHEN condition 3 THEN returnvalue3

……

ELSE defaultreturnvalue

END

举栗：

select t.name,(case when t.sex=0 then 'woman' when t.sex=1 then 'man' else 'unknown' end) sex from test t;

输出为：

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+-----------+-------+

| name | sex |

+-----------+-------+

| xiaohong | man |

| xiaoming | woman |

| xiaohuang | man |

| xiaoming | woman |

+-----------+-------+

2.if函数

IF(expr1,expr2,expr3)，如果 expr1 为真(expr1 <> 0 以及 expr1 <> NULL)，那么 IF() 返回 expr2，否则返回expr3。

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select name,if(weight>85,'fat','normal') from test;

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+-----------+------------------------------+

| name | if(weight>85,'fat','normal') |

+-----------+------------------------------+

| xiaohong | normal |

| xiaoming | fat |

| xiaohuang | normal |

| xiaoming | normal |

+-----------+------------------------------+

当weight为NULL时，NULL>85的返回值为NULL，所以返回expr3 normal

3.IFNULL()函数

ifnull(value1,value2) 用来替换 NULL 值，若value1为NULL，则替换NULL为value2

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select name,ifnull(weight,0) from test;

+-----------+------------------+

| name | ifnull(weight,0) |

+-----------+------------------+

| xiaohong | 50 |

| xiaoming | 90 |

| xiaohuang | 80 |

| xiaoming | 0 |

+-----------+------------------+

p.p1 { margin: 0; font: 16px Menlo; color: rgba(0, 0, 0, 1) }
span.s1 { font-variant-ligatures: no-common-ligatures }

select name,ifnull(weight,0)+100 from test;

+-----------+----------------------+

| name | ifnull(weight,0)+100 |

+-----------+----------------------+

| xiaohong | 150 |

| xiaoming | 190 |

| xiaohuang | 180 |

| xiaoming | 100 |

+-----------+----------------------+

4.ELT函数

ELT(N,str1,str2,str3,...)

如果 N = 1，返回 str1，如果N = 2，返回 str2，等等。如果 N 小于 1 或大于参数的数量，返回 NULL。

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select name,elt(level,'normal','vip','vvip') from test;

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+-----------+----------------------------------+

| name | elt(level,'normal','vip','vvip') |

+-----------+----------------------------------+

| xiaohong | normal |

| xiaoming | NULL |

| xiaohuang | vip |

| xiaoming | vvip |

+-----------+----------------------------------+

其他：

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，在该文中只讲述了if表达式。

参考：https://blog.csdn.net/rocling/article/details/82147981