一个SQL查询问题

给你个例子参考SQL> select * from tb1;       ID A B
--------- - --
        1 a 4
        2 a 3
        3 b 2
        4 c 1
        5 a 0SQL> select max(id),a from tb1 group by a;  MAX(ID) A
--------- -
        5 a
        3 b
        4 cSQL> select tb1.* from tb1,(select max(id) id,a from tb1 group by a) tb2 where tb1.id=tb2.id;       ID A B
--------- - --
        3 b 2
        4 c 1
        5 a 0