MySQL 删除表中重复的记录(where......)
2012-10-31 19:57
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delete from at1 where id in (select * from (select max(id) from at1 group by str having count(str) > 1) as b);
or
delete a from at1 as a, at1 as b where a.str=b.str and a.id > b.id;
3. 多个字段重复的问题。
“name”值存在重复的项; Select Name,Count(*) From A Group By Name Having Count(*) > 1
如果还查性别也相同大则如下: Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1
(见http://hi.baidu.com/yangjifu/blog/item/723e5f4cf92708ffd72afc92.html)
-----------------------------------------
1.MySQL的delete 支持:
DELETE t1,t2 FROM t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id
or
DELETE FROM t1,t2 USING t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id
2. 从 MySQL 4.0.4 开始,你也可以执行一个包含多个表的 UPDATE 的操作:
UPDATE items,month SET items.price=month.price
WHERE items.id=month.id;
注意:多表 UPDATE 不可以使用 ORDER BY 或 LIMIT。
3. 查询一个表中重复的记录:
select max(id) from at1 group by str having count(str) > 1
select a.id from at1 as a, at1 as b where a.str=b.str and a.id > b.id
4. delete from at1 where id in (select max(id) from at1 group by str having count(str) > 1); 是错误的:
ERROR 1093 : You can't specify target table 'at1' for update in FROM clause
正确写法:delete from at1 where id in (select * from (select max(id) from at1 group by str having count(str) > 1) as b);
理由(文档):
In general, you cannot modify a table and select from the same table in a subquery. For example, this limitation applies to statements of the following forms:
Exception: The preceding prohibition does not apply if you are using a subquery for the modified table in the
Here the prohibition does not apply because a subquery in the
mysql还支持 where (a,b) in (select a, b from aaaa)
or
delete a from at1 as a, at1 as b where a.str=b.str and a.id > b.id;
3. 多个字段重复的问题。
“name”值存在重复的项; Select Name,Count(*) From A Group By Name Having Count(*) > 1
如果还查性别也相同大则如下: Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1
(见http://hi.baidu.com/yangjifu/blog/item/723e5f4cf92708ffd72afc92.html)
-----------------------------------------
1.MySQL的delete 支持:
DELETE t1,t2 FROM t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id
or
DELETE FROM t1,t2 USING t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id
2. 从 MySQL 4.0.4 开始,你也可以执行一个包含多个表的 UPDATE 的操作:
UPDATE items,month SET items.price=month.price
WHERE items.id=month.id;
注意:多表 UPDATE 不可以使用 ORDER BY 或 LIMIT。
3. 查询一个表中重复的记录:
select max(id) from at1 group by str having count(str) > 1
select a.id from at1 as a, at1 as b where a.str=b.str and a.id > b.id
4. delete from at1 where id in (select max(id) from at1 group by str having count(str) > 1); 是错误的:
ERROR 1093 : You can't specify target table 'at1' for update in FROM clause
正确写法:delete from at1 where id in (select * from (select max(id) from at1 group by str having count(str) > 1) as b);
理由(文档):
In general, you cannot modify a table and select from the same table in a subquery. For example, this limitation applies to statements of the following forms:
DELETE FROM t WHERE ... (SELECT ... FROM t ...); UPDATE t ... WHERE col = (SELECT ... FROM t ...); {INSERT|REPLACE} INTO t (SELECT ... FROM t ...);
Exception: The preceding prohibition does not apply if you are using a subquery for the modified table in the
FROMclause. Example:
UPDATE t ... WHERE col = (SELECT (SELECT ... FROM t...) AS _t ...);
Here the prohibition does not apply because a subquery in the
FROMclause is materialized as a temporary table, so the relevant rows in
thave already been selected by the time the update to
ttakes place.
mysql还支持 where (a,b) in (select a, b from aaaa)
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