mysql中delete重复行,关于delete别名的用法

mysql> select * from tet7;
+----+------+
| id | t1   |
+----+------+
|  1 | 001  |
|  2 | 001  |
|  3 | 002  |
|  4 | 003  |
|  5 | 003  |
+----+------+
5 rows in set (0.00 sec)

删除重复值:
mysql> delete from tet7 where id  in (select a.id from (select max(id) as id from tet7  group by t1 having count(*)>1) a);
Query OK, 2 rows affected (0.06 sec)
ps:这个sql好理解,就是取出重复最大id值,然后根据这个id值删除该行数据... 注意,在子查询中,一定要将查出来的最大id,单独别名,在select这个别名表,否则会报语法错误.

以上是针对重复只有一个的..也就是相同的数据只有2行的情况..对于相同的行有多行的,有两种sql:

1.
mysql> select * from tet6;
+----+------+---------+
| id | d1   | d2      |
+----+------+---------+
|  1 |    1 | peng |
|  2 |    1 | peng |
|  3 |    2 | te      |
|  4 |    3 | te      |
|  5 |    3 | te      |
|  6 |    3 | te      |
|  7 |    4 | te      |
+----+------+---------+
7 rows in set (0.00 sec) mysql> delete  a
    ->  from tet6 as a,
    -> (
    -> select d1,d2,max(id) tt from tet6  group by d1,d2 having count(*) >1
    -> ) as b
    -> where a.id <b.tt and a.d1=b.d1 and a.d2=b.d2;
Query OK, 3 rows affected (0.04 sec)
ps:sql将原表作为a,将重复值的最大id及其重复的字段d1,d2共同作为b表.然后比较d1,d2值相当的值,但id小于最大id值,也可以是不等于,(大致意思就,取d1,d2值同时相等的,然后去id号不同的值),然后将其删除. 注意..在delete后面..只能取别名..不能写表名..不然会报错....

2.
delete from tet5
 where id not in (select id
                    from (select max(id) id
                            from tet5
                           group by name
                          having count(*) > 1) a)
   and id not in (select id
                    from (select max(id) id
                            from tet5
                           group by name
                          having count(*) = 1) b);

ps: 这个sql就相对好理解一些.第一个条件就是将重复值的最大id去除,第二个条件就是将不重复的值去除...剩下的就是需要删除的重复值.