提取表A.name中不存在于表B.name的值

mysql> desc A;
+----------------+-------------+------+-----+---------+----------------+
| Field          | Type        | Null | Key | Default | Extra          |
+----------------+-------------+------+-----+---------+----------------+
| id             | int(10)     | NO   | PRI | NULL    | auto_increment |
| uid            | int(10)     | NO   |     | NULL    |                |
| uname          | varchar(30) | NO   |     | NULL    |                |
| pwd            | varchar(20) | NO   |     | NULL    |                | |+----------------+-------------+------+-----+---------+---------------+

mysql> desc B;
+----------------+-------------+------+-----+---------+----------------+
| Field          | Type        | Null | Key | Default | Extra          |
+----------------+-------------+------+-----+---------+----------------+
| id             | int(10)     | NO   | PRI | NULL    | auto_increment |
| uname          | varchar(30) | NO   |     | NULL    |                |
|+----------------+-------------+------+-----+---------+---------------+

如果要提取表A.uname中不存在于表B.uname的值，其MySQL语句如下：

SELECT *
  FROM A  
  LEFT JOIN B ON A.uname=B.uname
  WHERE B.uname IS NULL