SQL多对多查询问题

Question

我有三个表：视频，视频类别和类别。

这些表如下所示：

videos: video_id, title, etc...
videos_categories: video_id, category_id
categories: category_id, name, etc...

在我的应用中，我允许用户多选类别。 当他们这样做时，我需要返回每个选定类别中的所有视频。

我结束了这个：

SELECT * FROM videos WHERE video_id IN (
    SELECT c1.video_id FROM videos_categories AS c1
    JOIN c2.videos_categories AS c2
    ON c1.video_id = c2.video_id
    WHERE c1.category_id = 1 AND c2.category_id = 2
)

但是对于我添加到多选中的每个类别，我必须向内部选择中添加一个联接：

SELECT * FROM videos WHERE video_id IN (
    SELECT c1.video_id FROM videos_categories AS c1
    JOIN videos_categories AS c2
    ON c1.video_id = c2.video_id
    JOIN videos_categories AS c3
    ON c2.video_id = c3.video_id
    WHERE c1.category_id = 1 AND c2.category_id = 2 AND c3.category_id = 3
)

我忍不住觉得这是做这件事的错误方法，但是我被阻止尝试寻找正确的方法。

Answer 1

如果这是主键：

 videos_categories: video_id, category_id

那么GROUP BY和HAVING应该可以工作，请尝试以下操作：

SELECT
    * 
    FROM videos 
    WHERE video_id IN (SELECT 
                           video_id
                           FROM videos_categories
                           WHERE category_id IN (1,2,3)
                           GROUP BY video_id
                           HAVING COUNT(video_id)=3
                      )

Answer 2

这是一个FOR XML PATH解决方案：

--Sample data
CREATE TABLE Video
    (
    VideoID int,
    VideoName varchar(50)
    )

CREATE TABLE Videos_Categories
    (
    VideoID int,
    CategoryID int
    )

INSERT  Video(VideoID, VideoName)
SELECT  1, 'Indiana Jones'
UNION ALL
SELECT  2, 'Star Trek'

INSERT Videos_Categories(VideoID, CategoryID)
SELECT 1, 1
UNION ALL
SELECT 1, 2
UNION ALL
SELECT 1, 3
UNION ALL
SELECT 2, 1
GO

--The query
;WITH   GroupedVideos
AS
(
SELECT  v.*,
        SUBSTRING(
                    (SELECT  (', ') + CAST(vc.CategoryID AS varchar(20))
                    FROM     Videos_Categories AS vc
                    WHERE    vc.VideoID = v.VideoID
                    AND      vc.CategoryID IN (1,2)
                    ORDER BY vc.CategoryID
                    FOR XML PATH('')), 3, 2000) AS CatList
FROM    Video AS v
)

SELECT  *
FROM    GroupedVideos
WHERE   CatList = '1, 2'

（忽略下面的所有内容-我读错了问题）

尝试

WHERE c1.category_id IN (1,2,3)

要么

...
FROM videos v
JOIN Vedeos_categories vc ON v.video_id = vc.video_id
WHERE vc.category_id IN (1,2,3)

根本不需要多个联接。

编辑：将解决方案放在上下文中（我知道这并不明显）：

SELECT * 
FROM videos 
WHERE video_id IN 
(    SELECT c1.video_id 
FROM videos_categories AS c1
WHERE c1.category_id = IN (1,2,3))

要么

SELECT *
FROM videos v
JOIN Vedeos_categories vc ON v.video_id = vc.video_id
WHERE vc.category_id IN (1,2,3)

Answer 3

听起来类似于SQL搜索包含多个条件的行

为了避免每个类别都需要另一个联接（从而更改查询的结构），可以将类别放入临时表中，然后针对该表进行联接。

CREATE TEMPORARY TABLE query_categories(category_id int);
INSERT INTO query_categories(category_id) VALUES(1);
INSERT INTO query_categories(category_id) VALUES(2);
INSERT INTO query_categories(category_id) VALUES(3);

SELECT * FROM videos v WHERE video_id IN (
  SELECT video_id FROM video_categories vc JOIN query_categories q ON vc.category_id = qc.category_id
  GROUP BY video_id
  HAVING COUNT(*) = 3
)

当然，尽管这很丑陋。 您可能要跳过临时表，而在子查询中只说'category_id IN（...）'。