面试遇到的问题 老猫

一个表，两个字段，A、B，取出每个A类型的B由大到小排列的前两个。

create table test_a
(
   a varchar2(10),
   b int
)
truncate table test_a;

insert into test_a values ('a',10);
insert into test_a values ('a',1);
insert into test_a values ('a',2);
insert into test_a values ('a',3);
insert into test_a values ('a',4);
insert into test_a values ('a',5);
insert into test_a values ('a',6);
insert into test_a values ('b',12);
insert into test_a values ('b',14);
insert into test_a values ('b',1);
insert into test_a values ('b',6);
insert into test_a values ('c',2);
insert into test_a values ('c',4);
insert into test_a values ('c',4);
insert into test_a values ('c',5);
insert into test_a values ('c',3);
insert into test_a values ('c',15);
insert into test_a values ('d',1);
insert into test_a values ('d',0);
insert into test_a values ('d',8);
insert into test_a values ('d',6);
insert into test_a values ('d',4);
insert into test_a values ('d',3);
commit;

--杨毅给出的答案如下 (没有使用分析函数)

--测试结果显示没显示去重，不过与题干无关。

SELECT *
  FROM TEST_A A
 WHERE A.B IN
       ((SELECT MAX(B.B) FROM test_a B WHERE A.A = B.A) UNION
        (SELECT MAX(C.B)
           FROM TEST_A C
          WHERE A.A = C.A
            AND C.B NOT IN
                (SELECT MAX(D.B) FROM TEST_A D WHERE A.A = D.A)))
 ORDER BY A.A ASC, A.B DESC;

--本人答案 （使用分析函数）

SELECT * FROM (
 SELECT a, b, ROW_NUMBER()
  OVER (
    PARTITION BY a ORDER BY b DESC
  ) Top2 FROM test_a
)
WHERE Top2 <= 2 ;