标签：create min 插入数据 class desc 最小 sel post too
mysql分组取每组前几条记录(排名) 附group by与order by的研究
http://www.jb51.net/article/31590.htm
--按某一字段分组取最大(小)值所在行的数据
/* 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：
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。
--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。
--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 /* name val memo ---------- ----------- -------------------- a 1 a1--a的第一个值 b 1 b1--b的第一个值 */
--三、按name分组取第一次出现的行所在的数据。
select a.* from tb a where val = (select top 1 val from tb where name = a.name) order by a.name /* name val memo ---------- ----------- -------------------- a 2 a2(a的第二个值) b 1 b1--b的第一个值 */
--四、按name分组随机取一条数据。
select a.* from tb a where val = (select top 1 val from tb where name = a.name order by newid()) order by a.name/* name val memo ---------- ----------- -------------------- a 1 a1--a的第一个值 b 5 b5b5b5b5b5 */
--五、按name分组取最小的两个(N个)val
select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val < a.val ) order by a.name,a.valselect a.* from tb a where val in (select top 2 val from tb where name=a.name order by val) order by a.name,a.val select a.* from tb a where exists (select count(*) from tb where name = a.name and val < a.val having Count(*) < 2) order by a.name /* name val memo ---------- ----------- -------------------- a 1 a1--a的第一个值 a 2 a2(a的第二个值) b 1 b1--b的第一个值 b 2 b2b2b2b2 */
--六、按name分组取最大的两个(N个)val
select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name,a.val select a.* from tb a where val in (select top 2 val from tb where name=a.name order by val desc) order by a.name,a.val select a.* from tb a where exists (select count(*) from tb where name = a.name and val > a.val having Count(*) < 2) order by a.name /* name val memo ---------- ----------- -------------------- a 2 a2(a的第二个值) a 3 a3:a的第三个值 b 4 b4b4 b 5 b5b5b5b5b5 */
--七，假如整行数据有重复，所有的列都相同（例如下表中的第5,6两行数据完全相同）。
按name分组取最大的两个(N个)val
/* 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 1 a1--a的第一个值 a 3 a3:a的第三个值 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 */
标签：create min 插入数据 class desc 最小 sel post too
原文地址：http://www.cnblogs.com/footleg/p/7133087.html
