开发经常犯的错误之→【join表连接关联查询 】

最近在审核SQL的时候，发现一些开发经常犯这个错误，下面我举个简单的例子，说明一下：

查找出A表和B表id相同的记录（并且把B表没有的id记录也查询出来），且name名字等于'b'的记录。

表数据：

mysql> select * from t1;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | d    |
|  5 | e    |
|  6 | b    |
+----+------+
6 rows in set (0.00 sec)
mysql> select * from t2;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | cc   |
|  4 | e    |
|  5 | b    |
+----+------+
5 rows in set (0.00 sec)

错误写法：

mysql> select * from t1 left join t2 on t1.id=t2.id and t1.name='b';
+----+------+------+------+
| id | name | id   | name |
+----+------+------+------+
|  2 | b    |    2 | b    |
|  1 | a    | NULL | NULL |
|  3 | c    | NULL | NULL |
|  4 | d    | NULL | NULL |
|  5 | e    | NULL | NULL |
|  6 | b    | NULL | NULL |
+----+------+------+------+
6 rows in set (0.00 sec)

正确写法：

mysql> select * from t1 left join t2 on t1.id=t2.id where t1.name='b';
+----+------+------+------+
| id | name | id   | name |
+----+------+------+------+
|  2 | b    |    2 | b    |
|  6 | b    | NULL | NULL |
+----+------+------+------+
2 rows in set (0.00 sec)

呵呵，可以当一道面试题了。