Sql 查找属于多个类别的项目

我有一个

projects

表（列

id

和

name

），一个

categories

表（列

id

和

name

）和一个join表

projects\u categories

（列

projectid

和

categories\u id

）

我想编写一个查询，返回属于一组类别的所有项目。例如，如果我有

projects id name 1 foo 2 bar 以下查询不返回任何结果：

SELECT "projects".* 
FROM "projects" 
INNER JOIN "projects_categories" ON "projects_categories"."project_id" = "project"."id" 
WHERE "projects_categories"."category_id" = 1
  AND "projects_categories"."category_id" = 2

---

我理解这些查询返回这些结果的原因，但不知道如何编写所需的查询。

您可以尝试使用

EXISTS

：

SELECT "p".* 
FROM "projects" "p" 
WHERE EXISTS ( 
    SELECT *
    FROM "projects_categories" "pc" 
    WHERE "pc"."category_id" IN (1, 2)
      AND "pc"."project_id" = "p"."id"
    GROUP BY "pc"."project_id" 
    HAVING  COUNT(DISTINCT "pc"."project_id" ) = 2)
)

或加入：

SELECT "p".* 
FROM "projects" "p" 
JOIN ( 
    SELECT "pc"."project_id" 
    FROM "projects_categories" "pc" 
    WHERE "pc"."category_id" IN (1, 2)
    GROUP BY "pc"."project_id" 
    HAVING  COUNT(DISTINCT "pc"."project_id" ) = 2)
) "tpc" ON "tpc"."project_id" = "p"."id"

SELECT "p".* 
FROM "projects" "p" 
WHERE "p"."id" IN ( 
    SELECT "pc"."project_id"
    FROM "projects_categories" "pc" 
    WHERE "pc"."category_id" IN (1, 2)
    GROUP BY "pc"."project_id" 
    HAVING  COUNT(DISTINCT "pc"."project_id" ) = 2)
)

或中的

：

SELECT "p".* 
FROM "projects" "p" 
JOIN ( 
    SELECT "pc"."project_id" 
    FROM "projects_categories" "pc" 
    WHERE "pc"."category_id" IN (1, 2)
    GROUP BY "pc"."project_id" 
    HAVING  COUNT(DISTINCT "pc"."project_id" ) = 2)
) "tpc" ON "tpc"."project_id" = "p"."id"

SELECT "p".* 
FROM "projects" "p" 
WHERE "p"."id" IN ( 
    SELECT "pc"."project_id"
    FROM "projects_categories" "pc" 
    WHERE "pc"."category_id" IN (1, 2)
    GROUP BY "pc"."project_id" 
    HAVING  COUNT(DISTINCT "pc"."project_id" ) = 2)
)

问题是，查询正在查找同一个表和列，并试图拥有两个不同的值。这将分离两个不同的条件，然后将它们组合成一个结果集

SELECT  A.ProjectName, A.CategoryId, B.CategoryId 
FROM    
(SELECT P.Name [ProjectName], PC.CategoryId, PC.ProjectId 
     FROM   @Projects P
            INNER JOIN @Project_Categories PC
                ON PC.ProjectId = P.ID
     WHERE  PC.CategoryId = 1
    ) A
    INNER JOIN (SELECT  P.Name [ProjectName], PC.CategoryId, PC.ProjectId
                FROM    @Projects P
                        INNER JOIN @Project_Categories PC
                            ON PC.ProjectId = P.ID
                WHERE   PC.CategoryId = 2
               ) B
        ON A.ProjectId = B.ProjectId

SELECT  A.ProjectName, A.CategoryId, B.CategoryId 
FROM    
(SELECT P.Name [ProjectName], PC.CategoryId, PC.ProjectId 
     FROM   @Projects P
            INNER JOIN @Project_Categories PC
                ON PC.ProjectId = P.ID
     WHERE  PC.CategoryId = 1
    ) A
    INNER JOIN (SELECT  P.Name [ProjectName], PC.CategoryId, PC.ProjectId
                FROM    @Projects P
                        INNER JOIN @Project_Categories PC
                            ON PC.ProjectId = P.ID
                WHERE   PC.CategoryId = 2
               ) B
        ON A.ProjectId = B.ProjectId