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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 
FROMtest 
WHEREid NOT IN (SELECTid FROM(SELECTid FROMtest WHEREid IN (SELECTid FROM( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a UNIONSELECTid 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 (SELECTcount(*) FROM(SELECTid FROMtest WHEREid IN (SELECTid FROM( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a UNIONSELECTid 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 
FROMtest 
WHEREid NOT IN (SELECTid FROM(SELECTid FROMtest WHEREid IN (SELECTid FROM( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a UNIONSELECTid FROM( SELECT max( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) b )) c ) AND (SELECTcount(*) FROM(SELECTid FROMtest WHEREid IN (SELECTid FROM( SELECT min( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) a UNIONSELECTid FROM( SELECT max( id ) id FROM `test` GROUP BY fd, fe HAVING count( id ) > 2 ORDER BY id ) b )) c ) > 0;