sql查询每个班级的前三名

1.建表语句，插入数据：

create table TEST_SCORE
(
    CLASS VARCHAR(10),
    NAME  VARCHAR(10),
    SCORE INTEGER
);

INSERT INTO test_score ( class, name, score) VALUES ( '1班', '齐静春', 100);
INSERT INTO test_score ( class, name, score) VALUES ( '1班', '李二', 99);
INSERT INTO test_score ( class, name, score) VALUES ( '1班', '陈平安', 88);
INSERT INTO test_score ( class, name, score) VALUES ( '1班', '李柳', 98);
INSERT INTO test_score ( class, name, score) VALUES ( '1班', '赵耀', 79);
INSERT INTO test_score ( class, name, score) VALUES ( '2班', '岳东江', 81);
INSERT INTO test_score ( class, name, score) VALUES ( '2班', '苏杰', 69);
INSERT INTO test_score ( class, name, score) VALUES ( '2班', '吴杰', 87);
INSERT INTO test_score ( class, name, score) VALUES ( '2班', '王珂', 77);
INSERT INTO test_score ( class, name, score) VALUES ( '2班', '杜兰特', 99);
INSERT INTO test_score ( class, name, score) VALUES ( '2班', '罗斯', 90);
INSERT INTO test_score ( class, name, score) VALUES ( '3班', '马祖号', 69);
INSERT INTO test_score ( class, name, score) VALUES ( '3班', '王祖辉', 89);
INSERT INTO test_score ( class, name, score) VALUES ( '3班', '林冲', 91);
INSERT INTO test_score ( class, name, score) VALUES ( '3班', '赵卡', 80);
INSERT INTO test_score ( class, name, score) VALUES ( '3班', '罗帅', 87);
INSERT INTO test_score ( class, name, score) VALUES ( '4班', '陈旭', 77);
INSERT INTO test_score ( class, name, score) VALUES ( '4班', '王艳', 71);
INSERT INTO test_score ( class, name, score) VALUES ( '4班', '呼呼', 89);
INSERT INTO test_score ( class, name, score) VALUES ( '4班', '陈帅', 78);

2.通用写法，不分数据库：

select class, name, score
from test_score a
where (select count(*) from test_score where class = a.class and a.score < score) < 3
order by a.class, a.score desc;

3.Oracle，DB2利用开口函数，

select class, name, score
from (select class, score, name,
             rank() over (partition by class order by score desc ) as rownum
      from test_score)
where rownum < 4;

4.MySQL利用变量实现，

set @pre_class:=null, @rownum:=0;
select
    class, name, score
from
    (SELECT class, score, name,
#             @rownum:=if(@pre_class=class,@rownum+1,1) count,
            @rownum:= (case when @pre_class = class then  @rownum + 1 else 1 end) count,
            @pre_class:=class
     FROM test_score
     order by class asc, score desc ) T
where count < 4;

5.查询结果如下，

参考：MYSQL:巧用临时变量做分组统计