Question

我有一个projects表（列id和name），一个categories表（列id和name）和一个加入表projects_categories（列project_id和category_id）。

我想编写一个返回属于一组类别的所有项目的查询。例如，如果我有

projects
id name
1  foo
2  bar

categories
id name
1  bee
2  gee

projects_categories
project_id category_id
1          1
1          2
2          1

我想编写一个返回项目的查询＆＃34; foo＆＃34;如果我传递类别ID 1和2.我尝试了以下查询，但它返回属于类别1或2的所有项目，而不是类别1和2。

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

以下查询不会返回任何结果：

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

我理解为什么这些查询会返回这些结果，但无法弄清楚如何编写我需要的查询。

Answer 1

您可以尝试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同一个表和列，并尝试使用两个不同的值。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

查找属于多个类别的项目

2 个答案: