Question

I am using MySQL. Let’s say I have table A, B and C. Table A is a base table. Table B and C both have foreign keys to table A. However I want to pull out the rows so that the output would look something like this.

A | B    | C
----------------
1 | 1    | NULL
1 | 2    | NULL
1 | NULL | 1
1 | NULL | 2
2 | 3    | NULL
2 | NULL | 3

So when B no longer has any rows left for a certain key the query moves on to C and stop outputting rows for B. This seems like it should be simple, but I seem to be having trouble coming up with the way it should be done.

Also the title of the question probably doesn’t make sense, but I wasn’t sure how to best describe the scenario.

Answer 1

Thinking about it, an OUTER JOIN isn’t going to give you the output format you want. Try something like this:

SELECT a.id AS A, b.id AS B, NULL AS C
FROM tableA a
INNER JOIN tableB ON a.id = b.fid
UNION
SELECT a.id AS A, NULL AS B, c.id AS C
FROM tableA a
INNER JOIN tableC ON a.id = c.fid
ORDER BY 1