Question

我有一个包含三列的数据库表。 Id，user_id，book_id。在此表中，有一些重复。 user_id应该只有一个book_id记录，但在某些情况下，user_id有几个book_id。已经有几百万条记录，我想知道如何删除任何重复记录。

Answer 1

尝试以下操作。

SQL SERVER

WITH ORDERED AS
(
    SELECT id
    ROW_NUMBER() OVER (PARTITION BY [user_id] , [book_id] ORDER BY id ASC) AS rn
    FROM
    tableName
)
delete from tableName
where id in ( select id from ORDERED where rn != 1)

MYSQL

delete from tableName
where id not in( 
    select MIN(id)from tableName    
    group by user_id, book_id
)

根据评论编辑 - 在MySQL中，您无法修改在SELECT部分中使用的同一个表

这将解决问题。

delete from tableName
where id not in( 
    select temp.temp_id from (
        select MIN(id) as temp_id from tableName    
        group by user_id, book_id
    ) as temp
)

这将只保留（user_id，book_id）

的一个组合

Answer 2

如果您执行以下声明，它将删除user_ID的所有重复记录，并为每个ID仅留下最大的user_ID

DELETE  a
FROM    tableName a
        LEFT JOIN
        (
            SELECT  user_ID, MAX(ID) max_ID
            FROM    tableName
            GROUP   BY user_ID
        ) b ON a.user_ID = b.user_ID AND
                a.ID = b.max_ID
WHERE   b.max_ID IS NULL

SQLFiddle Demo

Answer 3

希望此查询允许您删除重复项：

DELETE bl1 FROM book_log bl1
JOIN book_log bl2
ON (
    bl1.id > bl2.id AND
    bl1.user_id = bl2.user_id AND
    bl1.book_id = bl2.book_id
);

Demo

从关系数据库表中删除重复记录

3 个答案: