mysql去除重復數據

select可以取別名，delete不能。

1.使用mysql進行delete from操作時，若子查詢的 FROM 字句和更新/刪除對象使用同一張表，會出現錯誤。

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

針對“同一張表”這個限制，撇開效率不談，多數情況下都可以通過多加一層select 別名表來變通解決，像這樣

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

 

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2. mysql delete from where in 時後面 的查詢語句裡不能加where條件

 

Sql代碼  
      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代碼  
      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代碼  
      delete from `t_goods` where fi_id in ( select fi_id from `t_goods` )   

 上面三種情況，只有中間的不能執行。

綜合起來就是mysql delete from where in 時後面 的查詢語句裡不能加where條件

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3. delete from table... 這其中table不能使用別名

 Sql代碼  

    delete from student a where a.id in (1,2);(執行失敗)

    

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

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!!