SqlServer中 Partition By 的使用

1:数据库表结构：

（产品图片表）

select * from product_imgs

 

2:应用场景：

可见表中每一个产品(pro_id)可能对应多个图片(img)，我想要按照添加时间的先后顺序，获得其中每一个产品的第一张图片

3:尝试使用 group by 

select top 1 pro_id,img,create_time from product_imgs group by pro_id,img,create_time order by create_time 

发现无法满足每一个产品的图片

4:使用Partition by 

select * from (select pro_id,img,create_time, ROW_NUMBER() over(partition by pro_id order by  create_time) as row_sort  from product_imgs )
  as t where t.row_sort=1

 

可以满足需求，所以Partition by可以理解为 对多行数据分组后排序取每个产品的第一行数据

 

--20180712补充

最近又发现  cross apply 在联表的时候可以代替 partition by ,查询的效果是一样的

select a.id,a.title,t.* from Product as a
cross apply 
(select top 1 b.pro_id,b.img  from product_imgs as b where b.pro_id=a.id)
as t

--20190311补充

1.应用场景：查询学生最近一次各分类的体测成绩的分数

DECLARE @beginTime DATETIME = '2018-02-25 00:00:00'; -- datetime
DECLARE @endTime DATETIME = '2019-07-13 00:00:00'; -- datetime
DECLARE @schoolId VARCHAR(100) = '3301080020'; -- datetime
DECLARE @classId VARCHAR(100) = '33010800202018001'; -- datetime
DECLARE @groupId VARCHAR(100) = '761'; -- datetime
 --1.Cross APPLY + PARTITION by
 SELECT a.student_id,c.type,c.score,c.NewCol FROM dbo.Student  AS a
 CROSS APPLY
 (
    SELECT b.student_id,b.type,b.score,
	ROW_NUMBER() OVER(PARTITION BY b.type ORDER BY b.intime desc) AS NewCol
	FROM dbo.StudentTest AS b
	WHERE b.intime BETWEEN @beginTime AND @endTime
	AND type IN (1,2,3,6,7,8) AND a.student_id=b.student_id
	--GROUP BY b.student_id,b.type,b.score,b.intime
 )
 AS c
 WHERE a.class_id=@classId 
 AND c.NewCol=1
 

 --2.Left Join + PARTITION by
DECLARE @beginTime DATETIME = '2018-02-25 00:00:00'; -- datetime
DECLARE @endTime DATETIME = '2019-07-13 00:00:00'; -- datetime
DECLARE @schoolId VARCHAR(100) = '3301080020'; -- datetime
DECLARE @classId VARCHAR(100) = '33010800202018001'; -- datetime
DECLARE @groupId VARCHAR(100) = '761'; -- datetime
SELECT a.student_id,c.type,c.score,c.NewCol FROM dbo.Student  AS a
LEFT JOIN 
 (
    SELECT 
	b.student_id,b.type,b.score,
	ROW_NUMBER() OVER(PARTITION BY b.student_id,b.type ORDER BY b.intime desc) AS NewCol
	FROM dbo.StudentTest AS b
	WHERE b.intime BETWEEN @beginTime AND @endTime
	AND type IN (1,2,3,6,7,8)
 )
 AS c ON a.student_id=c.student_id
 WHERE a.class_id=@classId
 AND c.NewCol=1

2.两者区别：

CrossApply 由于在内指定了联表字段，故Partition By不用加StudentId分组，而Left Join 在外指定联表字段，故Partition By不用加StudentId分组

注：inner join 和 cross apply的不同，点击 https://blog.csdn.net/liuchang19950703/article/details/81015853