我继承了一个缺乏唯一约束的数据库,因此数据是重复的.我现在正在尝试删除重复的记录,然后添加一个约束来阻止这种情况发生.

我有这个问题：

SELECT count(*) as dacount, substr(group_concat(id), (locate(',', group_concat(id))+ 1))
FROM `game`
group by matchid, ordinal
having dacount > 1
order by dacount desc

这正确地给了我需要删除的行的ID.但问题是我不能在DELETE上使用它作为子查询,因为带有having参数的dacount.还有另一种方法吗？

这是我的计划：

DELETE FROM game WHERE id IN (SELECT count(*) as dacount, substr(group_concat(id), (locate(',', group_concat(id))+ 1))
FROM `game`
GROUP BY matchid, ordinal
HAVING dacount > 1)

解决方法:

MySQL的UPDATE和DELETE实现中存在关于自联接的限制.通常使用连接可以解决该问题：

DELETE g.*
FROM game AS g
  JOIN 
   ( SELECT matchid, ordinal, min(id) AS id 
     FROM game
     GROUP BY matchid, ordinal
     -- HAVING count(*) > 1
   ) AS d
   ON  g.matchid = d.matchid
   AND g.ordinal = d.ordinal
   AND g.id > d.id ;

在dbfiddle.uk测试.

另一种方法是使用LEFT JOIN / IS NULL结构：

DELETE g.*
FROM game AS g
  LEFT JOIN 
   ( SELECT min(id) AS id 
     FROM game
     GROUP BY matchid, ordinal
   ) AS d
   ON  g.id = d.id 
WHERE d.id IS NULL ;

或者将EXISTS子查询重写为JOIN
(删除存在具有相同matchid,序数和较小id的另一个游戏的所有游戏)：

DELETE g.*
FROM game AS g
  JOIN 
     game AS d
   ON  g.matchid = d.matchid
   AND g.ordinal = d.ordinal
   AND g.id > d.id
 ;