sql - Postgres多对多映射sql查询

Question

PostgreSQL 数据库

Table User
-----------
ID | Name

1  | John
2  | Bob
3  | Sarah


Table Photo
-------------
ID | Caption

1   | Vacation
2   | Birthday
3   | Christmas

Table Comment
--------------
ID | User ID | Photo ID| Text

1  |    1    |    1    | Mexico Looks Great
2  |    2    |    1    | Sure Does
3  |    3    |    1    | Too Hot
4  |    1    |    2    | Look at that cake
5  |    3    |    2    | No ice cream?
6  |    1    |    3    | So Happy

愿望:我想获得只有 John(1) 和 Sara(3) 评论过的所有照片。

我如何构建一个 SQL 查询来查找只有用户 #1 和用户 #3 的评论的照片，我想排除比这两个评论更多(或更少)的结果。

Answer 1

最清晰和最易读的方式是包含以下评论的照片:

User1 相交 User2 Except 任何其他用户

这SQL Fiddle查询将返回:

SELECT *
FROM Photo
WHERE ID IN (
    SELECT "Photo ID" FROM Comment WHERE "User ID" = 1
    INTERSECT
    SELECT "Photo ID" FROM Comment WHERE "User ID" = 3
    EXCEPT
    SELECT "Photo ID" FROM Comment WHERE "User ID" NOT IN (1, 3)
)