mysql流程控制函数

### if() 第一个值为true，返回第一个，否则第二个
mysql> select if(true,"123","456") from dual;
+----------------------+
| if(true,"123","456") |
+----------------------+
| 123                  |
+----------------------+

###ifnull() 第一个值不为空，返回第一个，第一个值为空，返回第二个
mysql> select ifnull("123","456") from dual;
+---------------------+
| ifnull("123","456") |
+---------------------+
| 123                 |
+---------------------+
1 row in set (0.00 sec)

mysql> select ifnull(null,"456") from dual;
+--------------------+
| ifnull(null,"456") |
+--------------------+
| 456                |
+--------------------+

### select case when expr then expr when expr then expr else expr end 哪一个成立返回哪个
mysql> select case when false then 1 when true then 2 else 3 end from dual;
+----------------------------------------------------+
| case when false then 1 when true then 2 else 3 end |
+----------------------------------------------------+
|                                                  2 |
+----------------------------------------------------+