Question

I have the following Tables:

Table A
id | name
1  | foo
2  | bar

Table B
id | name | default
1  | 123  | 1
2  | 321  | 0
3  | 456  | 1

Table C
A_id | B_id
1    | 2
1    | 3

now I would like to get the A-B pairs (with condition that these pairs have B.default = 1) which are not in C

So the expected result would be:

A_id | B_id
1    | 1
2    | 1
2    | 3

but I’m totally confused about how to join table B in:

SELECT A.id, B.id
FROM A

LEFT OUTER JOIN C
ON C.A_id = A.id

LEFT OUTER JOIN B
AND C.B_id = B.id
AND B.default = 1

Answer 1

Generate all A-pairs first, then compare to C

The NOT EXISTS is slightly more correct (anti-semi-join) than a LEFT JOIN

SELECT
    X.A_id, X.B_id
FROM
    (
    SELECT
      A.id AS A_id,
      B.id AS B_id
    FROM
      A CROSS JOIN B
    WHERE
      B.default = 1
    ) X
WHERE
   NOT EXISTS (SELECT *
      FROM
         C
      WHERE
         C.A_id = X.A_id
         AND
         C.B_id = X.B_id)

However, this gives this result. Only the C tuple of (1,3) matches the 4 A-B pairs

A_id | B_id
1    | 1
2    | 1
2    | 3