SQL Server：＆＃34; distinct＆＃34;的分页记录

Question

我有三个表格和列，如下所示

resource: id, location.
subject: id, name
resource_subject: resource_id, subject_id

＆＃34;资源＆＃34;之间的关系和＆＃34;主题＆＃34;是多对多的。这是我的查询：

select r.* from resource r 
inner join resource_subject subject on r.id = subject.resource_id 
where (subject.subject_id= 2 or subject.subject_id= 4) 

如上所述查询可能会产生重复记录，因为一个资源可能属于多个主题。因此，要删除重复项，我使用 distinct ，如下所示：

select distinct r.* from resource r 
inner join resource_subject subject on r.id = subject.resource_id 
where (subject.subject_id= 2 or subject.subject_id= 4) 

现在我想对独特记录进行分页。我读了这篇SO帖子，

What is the best way to paginate results in SQL Server

我对使用 ROW_NUMBER（）感兴趣。但是，使用ROW_NUMBER（）会使重复记录唯一：

select distinct ROW_NUMBER() over( order by r.id asc) AS rownum, r.* from resource r 
inner join resource_subject subject on r.id = subject.resource_id 
where (subject.subject_id= 2 or subject.subject_id= 4) 

如何在＃34; distinct＆＃34;之后对记录执行ROW_NUMBER（）？

Answer 1

使用DENSE_RANK()代替

ROW_NUMBER() is to SELECT what DENSE_RANK() is to SELECT DISTINCT。 DENSE_RANK()的以下应用程序将生成行号，就好像它们在应用DISTINCT后生成一样。

select distinct DENSE_RANK() over( order by r.id asc) AS rownum, r.* from resource r 
inner join resource_subject subject on r.id = subject.resource_id 
where (subject.subject_id= 2 or subject.subject_id= 4) 

它是如何运作的？

最好通过示例解释，也可以从linked article

解释

SELECT
  v, 
  ROW_NUMBER() OVER(ORDER BY v),
  RANK()       OVER(ORDER BY v),
  DENSE_RANK() OVER(ORDER BY v)
FROM (
  VALUES('a'),('a'),('a'),('b'),
        ('c'),('c'),('d'),('e')
) t(v);

以上产量：

| V | ROW_NUMBER | RANK | DENSE_RANK |
|---|------------|------|------------|
| a |          1 |    1 |          1 |
| a |          2 |    1 |          1 |
| a |          3 |    1 |          1 |
| b |          4 |    4 |          2 |
| c |          5 |    5 |          3 |
| c |          6 |    5 |          3 |
| d |          7 |    7 |          4 |
| e |          8 |    8 |          5 |

Answer 2

在外部查询中生成row_number

;WITH cte 
     AS (SELECT Row_number()OVER( ORDER BY id ASC) AS rownum, 
                * 
         FROM   (SELECT DISTINCT r.* 
                 FROM   resource r 
                        INNER JOIN resource_subject subject 
                                ON r.id = subject.resource_id 
                 WHERE  subject.subject_id IN ( 2, 4 )) A) 
SELECT * 
FROM   cte 
WHERE  rownum >= 1 
       AND rownum <= 10 

但如果您使用的是Sql Server 2012+，请使用OFFSET进行分页，这将比Row_number快

SELECT DISTINCT r.* 
FROM   resource r 
       INNER JOIN resource_subject subject 
               ON r.id = subject.resource_id 
WHERE  subject.subject_id IN ( 2, 4 ) 
ORDER  BY r.id 
offset 1 rows FETCH next 10 rows only;