删除重复数据，防止删除所有数据

这是mysql删除重复数据，且保留两条数据的sql,但这条语句含有一个危险的问题，如果子查询到的数据为空，会将表test的数据全部删除。

sql

DELETE 
FROM
    test 
WHERE
    id NOT IN (
        SELECT
            id 
        FROM
            (
                SELECT
                    id 
                FROM
                    test 
                WHERE
                    id IN (
                        SELECT
                            id 
                        FROM
                            ( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a 
                        UNION
                        SELECT
                            id 
                        FROM
                            ( SELECT max( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) b 
                    )
            ) c 
    ) 

思路

设置一个条件，这条件必须满足子查询有数据。

解决

在删除语句后，增加子查询有数据的结果大于0。

sql代码

 AND (
        SELECT
            count(*) 
        FROM
            (
                SELECT
                    id 
                FROM
                    test 
                WHERE
                    id IN (
                        SELECT
                            id 
                        FROM
                            ( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a 
                        UNION
                        SELECT
                            id 
                        FROM
                            ( SELECT max( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) b 
                    )
            ) c 
    ) > 0;    

完整代码

DELETE 
FROM
    test 
WHERE
    id NOT IN (
        SELECT
            id 
        FROM
            (
                SELECT
                    id 
                FROM
                    test 
                WHERE
                    id IN (
                        SELECT
                            id 
                        FROM
                            ( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a 
                        UNION
                        SELECT
                            id 
                        FROM
                            ( SELECT max( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) b 
                    )
            ) c 
    ) 
    AND (
        SELECT
            count(*) 
        FROM
            (
                SELECT
                    id 
                FROM
                    test 
                WHERE
                    id IN (
                        SELECT
                            id 
                        FROM
                            ( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a 
                        UNION
                        SELECT
                            id 
                        FROM
                            ( SELECT max( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) b 
                    )
            ) c 
    ) > 0;