专攻-针对一张表的详细SQL语句操作

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

tb表内容如下：

name	val	memo
a	1	a1--a的第1个值
a	2	a1--a的第2个值
a	3	a1--a的第3个值
b	1	b1--b的第1个值
b	2	b1--b的第2个值
b	3	b1--b的第3个值
b	4	b1--b的第4个值
b	5	b1--b的第5个值

执行结果

 

name	val	memo
a	3	a1--a的第3个值
b	5	b1--b的第5个值

二、按name分组随机取一条数据

 select a.* from tb a,(select top 1 * from tb order by newid()) b where a.name=b.name and a.val=b.val

三、按name分组取最小的2个(N个)val
 SELECT * FROM tb a WHERE a.val IN (SELECT TOP 2 val FROM tb where name=a.name ORDER BY val) order BY a.name
 SELECT * FROM tb a WHERE 2>(SELECT COUNT(*) from tb where name=a.name AND val<a.val) ORDER BY a.name,a.val
 SELECT * FROM tb a WHERE EXISTS (SELECT COUNT(*) from tb b where b.name=a.name and b.val < a.val HAVING count(*) < 2) ORDER BY a.name,a.val