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(1a931f5fdc237368def39403338980da0 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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