select case when语句

今天看见一公司的数据库面试题，其中有道一开始没想起怎么做，后来摸索了下终于做出来了。

题目是：两个表联合查询，当表2的记录在表A里没有时，将其设置为0.

mysql> select * from t1;
+----+------+
| id | name |
+----+------+
|  1 | bbs  |
|  2 | bb   |
|  3 | cc   |
+----+------+
3 rows in set (0.01 sec)

mysql> select * from t2;
+----+------+
| id | name |
+----+------+
|  1 | aa   |
|  2 | bb   |
+----+------+
2 rows in set (0.00 sec)

mysql> select t1.name as name1,t2.name as name2 from t1 left join t2 on t1.id=t2.id;                                        
+-------+-------+
| name1 | name2 |
+-------+-------+
| bbs   | aa    |
| bb    | bb    |
| cc    | NULL  |
+-------+-------+
3 rows in set (0.00 sec)

将NULL变为0

第一种写法：

mysql> select t1.name as name1,if(t2.name is NULL,0,t2.name) as name2 from t1 left join t2 on t1.id=t2.id;                        
+-------+-------+
| name1 | name2 |
+-------+-------+
| bbs   | aa    |
| bb    | bb    |
| cc    | 0     |
+-------+-------+
3 rows in set (0.00 sec)
 

第二种写法：

mysql> select t1.name as name1,(case when t2.name is NULL then '0' else t2.name end) as name2 from t1 left join t2 on t1.id=t2.id;  
+-------+-------+
| name1 | name2 |
+-------+-------+
| bbs   | aa    |
| bb    | bb    |
| cc    | 0     |
+-------+-------+
3 rows in set (0.00 sec)

参考：

http://apps.hi.baidu.com/share/detail/22393623