查找属于多个类别的项目

Question

我有一个 projects table（id 和 name 列），一个 categories table（[=13= 列] 和 name) 和连接 table projects_categories（列 project_id 和 category_id）。

我想编写一个查询，其中 return 属于一组类别的所有项目。例如，如果我有

项目
身份证名称
1 富
2巴

类别
身份证名称
1只蜜蜂
2 哎呀

projects_categories
project_id category_id
1 1
1 2
2 1

如果我传递类别 ID 1 和 2，我想编写一个查询 return 将项目 "foo" 告诉我。我尝试了以下查询，但它 returns 属于类别 1 或 2，而不是类别 1 和 2 的所有项目。

SELECT "projects".* 
FROM "projects" 
INNER JOIN "projects_categories" ON "projects_categories"."project_id" = "project"."id" 
WHERE "projects_categories"."category_id" IN (1, 2)

以下查询没有return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

我理解为什么这些查询 return 这些结果，但不知道如何编写我需要的查询。

Answer 1

你可以试试 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JOIN：

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"

或IN：

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)
)

Answer 2

问题是查询正在查看相同的 table 和列并试图获得两个不同的值。这样就把两个不同的条件分开，然后合并成一个结果集。

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

查找属于多个类别的项目

Find projects which belong to several categories

sql

postgresql