mysql分组取每组前几条记录分组取每组前几条记录(排名排名) 附附group by与与order by的的
研究研究
–按某一字段分组取最大(小)值所在行的数据
代码如下:
/*
数据如下：
name val memo
a 2 a2(a的第二个值)
a 1 a1–a的第一个值
a 3 a3:a的第三个值
b 1 b1–b的第一个值
b 3 b3:b的第三个值
b 2 b2b2b2b2
b 4 b4b4
b 5 b5b5b5b5b5
*/
–创建表并插入数据：
代码如下:
create table tb(name varchar(10),val int,memo varchar(20))
insert into tb values(‘a’, 2, ‘a2(a的第二个值)’)
insert into tb values(‘a’, 1, ‘a1–a的第一个值’)
insert into tb values(‘a’, 3, ‘a3:a的第三个值’)
insert into tb values(‘b’, 1, ‘b1–b的第一个值’)
insert into tb values(‘b’, 3, ‘b3:b的第三个值’)
insert into tb values(‘b’, 2, ‘b2b2b2b2’)
insert into tb values(‘b’, 4, ‘b4b4’)
insert into tb values(‘b’, 5, ‘b5b5b5b5b5’)
go
–一、按name分组取val最大的值所在行的数据。
代码如下:
–方法1：select a.* from tb a where val = (select max(val) from tb where name = a.name) order by a.name
–方法2：
select a.* from tb a where not exists(select 1 from tb where name = a.name and val > a.val)
–方法3：
select a.* from tb a,(select name,max(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by
a.name
–方法4：
select a.* from tb a inner join (select name , max(val) val from tb group by name) b on a.name = b.name and a.val = b.val
order by a.name
–方法5
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name
/*
name val memo
———- ———– ——————–
a 3 a3:a的第三个值
b 5 b5b5b5b5b5
*/
本人推荐使用1,3,4,结果显示1,3,4效率相同，2，5效率差些，不过我3,4效率相同毫无疑问，1就不一样了，想不搞了。
–二、按name分组取val最小的值所在行的数据。
代码如下:
–方法1：select a.* from tb a where val = (select min(val) from tb where name = a.name) order by a.name
–方法2：
select a.* from tb a where not exists(select 1 from tb where name = a.name and val < a.val)
–方法3：
select a.* from tb a,(select name,min(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by
a.name
–方法4：
select a.* from tb a inner join (select name , min(val) val from tb group by name) b on a.name = b.name and a.val = b.val
order by a.name
–方法5
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val < a.val) order by a.name
/*
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