Postgres Select from a Table Based On Query Result - postgresql

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 )
;

Answer 2

SQL syntax does not allow conditional joins.
Probably the simplest way to achieve a similar effect is to use a dynamic query in a plpgsql function, which may look like this:
create function conditional_select(acid int, some_condition boolean)
returns table (aid int, bid int, cid int)
language plpgsql as $$
declare
tname text;
begin
if some_condition then tname = 'a1';
else tname = 'a2';
end if;
return query execute format ($fmt$
select a.aid, b.bid, b.cid
from %s a
join b using(bid)
where cid = %s;
$fmt$, tname, acid);
end $$;
select * from conditional_select(1, true)

Answer 3

If, like in your example, you have only a few columns that you want to output, you can use the CASE statement for every column:
SELECT CASE z.someCondition THEN a1.aid ELSE a2.aid END AS aid,
CASE z.someCondition THEN a1.bid ELSE a2.bid END AS bid
FROM b
JOIN a1 ON a1.bid = b.bid
JOIN a2 ON a2.bid = b.bid
JOIN someTable z USING (cid);
Depending on the size of tables a1 and a2 and how many columns you have to output, this may or my not be faster than Klin's solution with a function, which is inherently slower than plain SQL and even more so because of the dynamic query. Given that z.someCondition is a boolean value already, the CASE evaluation will be very fast. Small tables + few columns = this solution; large tables + many columns = Klin's solution.