Get count of foreign key from multiple tables

Question

I have 3 tables, with Table B & C referencing Table A via Foreign Key. I want to write a query in PostgreSQL to get all ids from A and also their total occurrences from

Answer 1

Use left join with a subquery:

SELECT a.id, COUNT(x.id)
FROM a
LEFT JOIN (
    SELECT id, a_id FROM b
    UNION ALL
    SELECT id, a_id FROM c
) x ON (a.id = x.a_id)
GROUP BY a.id;