Postgres Select from a Table Based On Query Result

Question

I have two tables with identical columns, in an identical order. I have a desire to join across one of the two tables, depending on a subquery condition. For example, assume I have the following schema:

CREATE TABLE b (
    bid SERIAL PRIMARY KEY,
    cid INT NOT NULL
);

CREATE TABLE a1 (
    aid SERIAL PRIMARY KEY,
    bid INT NOT NULL REFERENCES b
);

CREATE TABLE a2 (
    aid SERIAL PRIMARY KEY,
    bid INT NOT NULL REFERENCES b
);

I would like a query, that performs a join across either a1 or a2 based on some condition. Something like:

WITH z AS (
  SELECT cid, someCondition FROM someTable
)
SELECT *
FROM CASE z.someCondition THEN a1 ELSE a2 END
JOIN b USING (bid)
WHERE cid = (SELECT cid FROM z);

However, the above doesn't work. Is there some way to conditionally join across a1 or a2, depending on some boolean condition stored in table z?

Answer 1

If the conditions are exclusive (I expect they are): just do both queries and UNION ALL them, with the smart union construct:

WITH z AS (
  SELECT cid
        , (cid %3) AS some_condition -- Fake ... 
        FROM  b
  )
SELECT *
  FROM a1
  JOIN b USING (bid)
 WHERE EXISTS( SELECT * FROM z
        WHERE some_condition = 1 AND cid = b.cid )
UNION ALL
SELECT *
  FROM a2
  JOIN b USING (bid)
 WHERE EXISTS( SELECT * FROM z
        WHERE some_condition = 2 AND cid = b.cid )
        ;

---

A somewhat different syntax to do the same:

WITH z AS (
  SELECT cid
        , (cid %3) AS some_condition 
        FROM  b
)
SELECT *
  FROM a1
  JOIN b ON a1.bid = b.bid
        AND EXISTS( SELECT * FROM z
        WHERE some_condition = 1 AND cid = b.cid )
UNION ALL
SELECT *
  FROM a2
  JOIN b ON a2.bid = b.bid
        AND EXISTS( SELECT * FROM z
        WHERE some_condition = 2 AND cid = b.cid )
        ;