查找、删除重复记录

用法一: 查找很快，用到Group by语句
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。-----小心使用

用法二: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,需要用到这个办法:
---- 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);