sql同表分组取第一条

一、使用第一种，发现速度慢

SELECT   CategoryPermission.*,
P.LevelNum,
    Flag = CAST(0 AS TINYINT) INTO #Permission FROM CategoryPermission P 
    OUTER APPLY
(select TOP(1) EmployeeId ,
        CategoryId ,
        BrandId,
        RoleType from CategoryPermission   WHERE EmployeeId =p.EmployeeId and CategoryId=p.CategoryId and BrandId=p.BrandId   ORDER BY StartTime,[EndTime] desc)CategoryPermission;

二、改为第二种（参考文章 https://www.jb51.net/article/34655.htm）

SELECT    P.*,
    Flag = CAST(0 AS TINYINT) INTO #Permission  from
(select CategoryPermission.* from (select n =row_number() over (partition by EmployeeId, CategoryId ,BrandId order by StartTime,EndTime desc),
 EmployeeId ,
        CategoryId ,
        BrandId ,
        LevelNum,
        RoleType
        from CategoryPermission )CategoryPermission
         where CategoryPermission.n = 1)P