SQL-从两个表中获取元素，第三个表包含所有行

Question

I am currently trying to create a SQL statement that allows me to retrieve information from two tables, with a third one which has the post_id from each table and the type. This will allow me to know which table the post_id is from.

ALL_POSTS:

id PRIMARY KEY,
post_id (FOREIGN KEY)
type

MOVIES:

post_id
type
title

MUSIC:

post_id
type
title
band

This is the SQL statement I used:

SELECT a.*, b.*, c.* FROM ALL_POSTS as a, MOVIES as b, MUSIC as c WHERE (a.post_id = b.id AND a.type = b.type) OR (a.post_id = c.id AND a.type = c.type)

The problem is that it retrieves a lot of results, and all of them are from the MUSIC table, and most of them are repeated.

Thanks

Answer 1

You should use a LEFT JOIN on both tables. This way, it would return all the rows on the ALL_POSTS table and the corresponding details on the MOVIES and MUSIC table.

SELECT * FROM ALL_POSTS AS A
    LEFT JOIN MOVIES AS B ON A.post_id = B.post_id AND A.type = B.type
    LEFT JOIN MUSIC AS C ON A.post_id = B.post_id AND A.type = C.type

Answer 2

At the moment you are generating a cartesian product from the three tables. Use JOIN s instead:

SELECT A.*, B.*, C.* FROM ALL_POSTS AS A
LEFT JOIN MOVIES AS B ON A.post_id = B.ID AND A.type = B.type
LEFT JOIN MUSIC AS c ON A.post_id = C.ID AND A.type = C.type