[TOC]
MySQL中进行条件判断的函数有IF、IFNULL和CASE。
1.IF(expr,v1,v2)函数
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 (0.05 sec)
说明:
IF(expr,v1,v2) expr结果为true则返回v1,结果为false则返回v2。
2.IFNULL(v1,v2)函数
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)
说明:
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。
3.CASE函数
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)
mysql> SELECT CASE WHEN 1<0 THEN 'true' ELSE 'false' END;
+--------------------------------------------+
| CASE WHEN 1<0 THEN 'true' ELSE 'false' END |
+--------------------------------------------+
| false |
+--------------------------------------------+
1 row in set (0.00 sec)
说明:
1.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,如果都不匹配,则返回ELSE后面的rn
2.CASE WHEN v1 THEN r1[WHEN v2 THEN r2] ELSE rn END
该函数表示某个vn值为true时,返回对应位置THEN后面的结果,如果所有值都不为TRUE,则返回ELSE后的rn。