这条语句有什么东东啊

这条语句有什么错误啊？
我是删除数据库里 有某些字段相同的记录只保留一个，这样怎么做？

SQL code

DELETE FROM aFROM         TollData AS a CROSS JOIN                      aWHERE     EXISTS                          (SELECT     1 AS Expr1                            FROM          TollData                            WHERE      (CardID = a.CardID) AND (StCardDate = a.starddate) AND (Balance = a.Balance))

SQL code

delete from a where id not in (select max(id) from a group by CardID,StCardDate ,Balance )