单表删除语句：
DELETE [LOW_PRIORITY][IGNORE] FROM tbl_name
[WHERE where_definition] [ORDER BY ...] [LIMIT row_count]这里只对delete的常用参数及用法做出介绍。
说明：
low_priority：强制delete命令被延迟执行，直到没有其它客户端读取本表时再执行。
ignore：使MySQL忽略delete执行所有的错误，而错误会以警告的方式返回。
order by...：表中记录按照指定的顺序进行删除，此子句一般与limit联用才有意义。
limit：指定删除操作的指定行范围。
注意：1）不能把ORDER BY或LIMIT与multiple-table UPDATE（多表更新）同时使用；
2）当delete伴随有多个限制条件时，是将多个限制条件执行查询操作后，再执行对应的删除操作。
（1）简单单表删除
语句：delete from table_name where [条件]
示例：
mysql> select * from staff;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| | guoding | |
| | dingtao | |
| | haofugui | |
| | guoming | |
| | haotian | |
| | fengfei | |
| | guoting | |
| | guoding | |
| | xiaohua | |
+----+----------+-------+
rows in set (0.00 sec) mysql> delete from staff where id=9; //通过where指定要删除的记录或记录组合
Query OK, row affected (0.00 sec) mysql> select * from staff;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| | guoding | |
| | dingtao | |
| | haofugui | |
| | guoming | |
| | haotian | |
| | fengfei | |
| | guoting | |
| | guoding | |
+----+----------+-------+
rows in set (0.00 sec)
（2）删除-排序、限制
order by...一般与limit搭配使用才有意义。
如：删除工资最低的记录
mysql> select * from staff;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| | guoding | |
| | dingtao | |
| | haofugui | |
| | guoming | |
| | haotian | |
| | fengfei | |
| | guoting | |
| | guoding | |
+----+----------+-------+
rows in set (0.00 sec) mysql> delete from staff order by slary asc limit 1; //删除工资最低的数据记录
Query OK, row affected (0.00 sec) mysql> select * from staff;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| | guoding | |
| | dingtao | |
| | haofugui | |
| | guoming | |
| | haotian | |
| | fengfei | |
| | guoding | |
+----+----------+-------+
rows in set (0.00 sec)
（3）连接删除
假设有一个post表与staff对应：
mysql> select * from staff;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| | guoding | |
| | dingtao | |
| | haofugui | |
| | guoming | |
| | haotian | |
| | fengfei | |
| | guoding | |
+----+----------+-------+
rows in set (0.00 sec) mysql> select * from post;
+-----+----------+-------+
| num | name | level |
+-----+----------+-------+
| | haotian | |
| | guoming | |
| | haofugui | |
+-----+----------+-------+
rows in set (0.00 sec)
现在需要从表staff中删除在post没有对应记录的记录：
mysql> delete staff from staff left join post on staff.name=post.name where post.name is NULL;
Query OK, rows affected (0.00 sec) mysql> select * from staff;
+----+----------+-------+
| id | name | slary |
+----+----------+-------+
| | haofugui | |
| | guoming | |
| | haotian | |
+----+----------+-------+
rows in set (0.00 sec)
在上述删除命令中，也可以引入别名，方便输入，如：
mysql> delete A from staff as A left join post as B on A.name=B.name where B.name is NULL;
注意：NULL不可以用等号（B.name=NULL），正确的应该是B.name is NULL