MySQL条件判断函数

1、IF(expr, v1, v2)函数

如果expr成立，返回值为v1，否则返回v2

mysql> select if(1>2,2,3), -> if(1<2,'yes','no'), -> if(strcmp('test','test1'),'no','yes');
+-------------+--------------------+---------------------------------------+| if(1>2,2,3) | if(1<2,'yes','no') | if(strcmp('test','test1'),'no','yes') |
+-------------+--------------------+---------------------------------------+| 3 | yes | no |
+-------------+--------------------+---------------------------------------+1 row in set (0.00 sec)

2.IFNULL(v1, v2)函数

IFNULL(v1,v2)假如v1不为NULL，则IFNULL()的返回值为v1；否则其返回值为v2。IFNULL()的返回值是数字或是字符串，具体情况取决于其所在的语境。

mysql> select ifnull(1,2),ifnull(NULL,10),ifnull(1/0,'wrong');
+-------------+-----------------+---------------------+| ifnull(1,2) | ifnull(NULL,10) | ifnull(1/0,'wrong') |
+-------------+-----------------+---------------------+|           1 |              10 | wrong               |
+-------------+-----------------+---------------------+1 row in set (0.00 sec)

3.CASE函数

CASE expr WHEN v1 THEN r1 [WHEN v2 THEN r2] [ELSE rn] END
该函数表示，如果expr值等于某个vn，则返回对应位置THEN后面的结果。如果与所有值都不相等，则返回ELSE后面的rn。

mysql> select case 2 when 1 then 'one' when 2 then 'two' else 'more' end;
+------------------------------------------------------------+| case 2 when 1 then 'one' when 2 then 'two' else 'more' end |
+------------------------------------------------------------+| two                                                        |
+------------------------------------------------------------+1 row in set (0.00 sec)

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