查找、删除重复记录
查找、删除重复记录: 法一: 用Group by语句 此查找很快的 select count(num), max(name) from student --查找表中num列重复的，列出重复的记录数，并列出他的name属性 group by num having count(num) >1 --按num分组后找出表中num列重复，即出现次数大于一次 delete from student(上面Select的) 这样的话就把所有重复的都删除了。-----慎重 法二:当表比较大(例如10万条以上)时,这个方法的效率之差令人无法忍受,需要另想办法: ---- 执行下面SQL语句后就可以显示所有DRAWING和DSNO相同且重复的记录 SELECT * FROM EM5_PIPE_PREFAB WHERE ROWID!=(SELECT MAX(ROWID) FROM EM5_PIPE_PREFAB D --D相当于First,Second WHERE EM5_PIPE_PREFAB.DRAWING=D.DRAWING AND EM5_PIPE_PREFAB.DSNO=D.DSNO); ---- 执行下面SQL语句后就可以刪除所有DRAWING和DSNO相同且重复的记录 DELETE FROM EM5_PIPE_PREFAB WHERE ROWID!=(SELECT MAX(ROWID) FROM EM5_PIPE_PREFAB D WHERE EM5_PIPE_PREFAB.DRAWING=D.DRAWING AND EM5_PIPE_PREFAB.DSNO=D.DSNO);
?
