加入的,而不是相关子查询(Join instead of correlated subquery)

2019-10-18 00:12发布

CREATE TABLE BlogPosts 
(
PostID INT PRIMARY KEY  not null,
PostTitle NVARCHAR ,
BlogID int,
TotalComments int
)

可这个查询简化与任何加入相关子查询的呢?

SELECT TOP 5 *
FROM BlogPosts as t0
WHERE t0.PostID = (SELECT TOP 1 t1.PostID 
                FROM BlogPosts as t1
                WHERE t0.BlogID = t1.BlogID
                ORDER BY t1.TotalComments DESC)

我需要从不同的博客5个员额,最大TotalComments。

UPD。 SQL Server中,但我宁愿标准的SQL

Answer 1:

如果我理解正确的,是帖子ID唯一的,所以这应该帮助

编辑:

OK试试这个,然后

DECLARE @BlogPosts  TABLE
( 
    PostID INT PRIMARY KEY  not null, 
    PostTitle NVARCHAR , 
    BlogID int, 
    TotalComments int 
) 

INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 1, 'A', 1, 3
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 2, 'B', 1, 4
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 3, 'C', 2, 5
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 4, 'D', 2, 6
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 5, 'E', 2, 7
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 6, 'F', 1, 8
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 7, 'G', 3, 9
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 8, 'H', 4, 10
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 9, 'I', 5, 11
INSERT INTO @BlogPosts (PostID,PostTitle,BlogID,TotalComments) SELECT 10, 'J', 6, 5

SELECT  TOP 5 *
FROM    @BlogPosts bp INNER JOIN
        (
            SELECT  BlogID,
                    MAX(TotalComments) MaxComments
            FROM    @BlogPosts
            GROUP BY BlogID
        ) maxCommentsPerBlog ON bp.BlogID = maxCommentsPerBlog.BlogID
                            AND bp.TotalComments = maxCommentsPerBlog.MaxComments
ORDER BY bp.TotalComments DESC

你可能会虽然有多个最大博客-totalComments组合。



Answer 2:

这会给你每博客上一篇:

SELECT * FROM (
  SELECT *, Ranking = ROW_NUMBER() OVER (PARTITION BY BlogID ORDER BY TotalComments DESC)
  FROM BlogPosts
  ) a
WHERE Ranking = 1

或者:

SELECT b.*
FROM (
  SELECT DISTINCT BlogID 
  FROM BlogPosts 
  ) a
CROSS APPLY (
  SELECT TOP 1 b.* FROM BlogPosts b
  WHERE a.BlogID = b.BlogID
  ORDER BY b.TotalComments DESC
  ) b

这是你在找什么?



Answer 3:

为什么你需要连接和子查询? 为什么你不能写

SELECT  TOP 5 *
FROM    @BlogPosts bp order by TotalComments desc;


文章来源: Join instead of correlated subquery