mysql去除重复数据_MySQL



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select可以取别名，delete不能。

1.使用mysql进行delete from操作时，若子查询的 FROM 字句和更新/删除对象使用同一张表，会出现错误。

mysql> DELETE FROM tab1 WHERE col1 = ( SELECT MAX( col1 ) FROM tab1 );
ERROR 1093 (HY000): You can’t specify target table ‘tab1′ for update in FROM clause

针对“同一张表”这个限制，撇开效率不谈，多数情况下都可以通过多加一层select 别名表来变通解决，像这样

DELETE FROM tab1
WHERE col1 = (
SELECT MAX( col1 )
FROM (
SELECT * FROM tab1
) AS t
);

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2. mysql delete from where in 时后面 的查询语句里不能加where条件

Sql代码
delete from `t_goods` where fi_id in (select * from ( select fi_id from `t_goods` where fs_num is null and fs_name is null and fs_type is null andfs_using is null and fs_lifetime is null) b) Sql代码
delete from `t_goods` where fi_id in (select fi_id from `t_goods` where fs_num is null and fs_name is null and fs_type is null andfs_using is null and fs_lifetime is null) Sql代码
delete from `t_goods` where fi_id in ( select fi_id from `t_goods` )

上面三种情况，只有中间的不能执行。

综合起来就是mysql delete from where in 时后面 的查询语句里不能加where条件

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3. delete from table... 这其中table不能使用别名

Sql代码

delete from student a where a.id in (1,2);(执行失败)

select a.* from student a where a.id in (1,2);(执行成功)

DELETE FROM v9_news WHERE title IN (SELECT title FROM (SELECT title FROM v9_news as newsb WHERE FROM_UNIXTIME(newsb.inputtime,'%Y-%m-%d')='2013-06-27' GROUP BY newsb.title HAVING COUNT(*) >1 ) b)
AND id NOT IN (SELECT id FROM (SELECT MIN(id) AS id FROM v9_news WHERE FROM_UNIXTIME(inputtime,'%Y-%m-%d')='2013-06-27' GROUP BY title HAVING COUNT(*)>1) c); TRUE!!
DELETE FROM v9_news AS news
WHERE news.title IN (SELECT title FROM v9_news as newsb WHERE FROM_UNIXTIME(newsb.inputtime,'%Y-%m-%d')='2013-06-27' GROUP BY newsb.title HAVING COUNT(*) >1 )
AND id NOT IN (SELECT MIN(id) FROM v9_news WHERE FROM_UNIXTIME(inputtime,'%Y-%m-%d')='2013-06-27' GROUP BY title HAVING COUNT(*)>1); FALSE!!

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