数据库分组筛选

分组筛选的基本思路是增加行号字段。
sqlserver分组行号添加方法：

select * from

(

select ROW_NUMBER()over(PARTITION By OrderName ORDER BY RequestDate DESC) as rownum, OrderCount,OrderName 

from #Order1

) T

where T.rownum = 1
mysql分组行号添加方法：

SELECT t.a, t.b
    FROM (SELECT f.a,f.b, @rank:=IF(@b<>f.b,IF(@a=f.a, @rank+1, 1),@rank) rank, @a:=f.a, @b:=f.b
         FROM (SELECT a,b FROM `SAM` ORDER BY a,b) f, (SELECT @rank:=1,@a:=null,@b:=null) tt ) t
    WHERE rank=3;