mysql select with if condition - php

Question

I have a function in Mysql that is called like this:
DB::raw("count_adults(rooms.id) as adults"),
count_adults is this:
BEGIN
DECLARE adults INT;
SELECT
count(*) INTO adults
FROM
clients
WHERE
clients.room_id = r_id
and (age >= 18
or age = 0);
RETURN adults;
END
However I want to if this query returns no results(0) do another select or function, like this:
SELECT count(*) INTO adults FROM client_room, clients
WHERE client_id = clients.id and client_room.room_id = r_id and (age >18 or age = 0)
Can I create a if condition? IN the php code? Or in the query? Something like this:
IF EXISTS (SELECT 1 FROM clients WHERE clients.room_id = r_id)
BEGIN
DECLARE adults INT;
SELECT
count(*) INTO adults
FROM
clients
WHERE
clients.room_id = r_id
and (age >= 18
or age = 0);
RETURN adults;
END
ELSE
BEGIN
DECLARE adults INT;
SELECT count(*) INTO adults
FROM
client_room, clients
WHERE
client_id = clients.id and client_room.room_id = r_id
and (age >18
or age = 0);
END
I have tried this and it seems to work:
BEGIN
DECLARE adults INT;
SELECT COUNT(*) INTO adults
FROM clients
WHERE clients.room_id = r_id
and (age >= 18 or age = 0);
RETURN IF( adults>0, adults, (SELECT count(*) FROM client_room, clients
WHERE client_id = clients.id
and client_room.room_id = r_id)
);
END

Answer 1

You could use CASE in the query, e.g.:
CASE
WHEN function1() > 0
THEN function1()
WHEN function2() > 0
THEN function2()
ELSE
0
END
That will evaluate to the value returned from function1() if it is larger than 0. If it's not but the value returned by function2() is larger then 0, then this is taken. Otherwise it's 0.
Instead of function1(), function2() etc. a column, subquery (that is, if it returns a scalar), literal, etc. can be used. And of course it can be changed to more or less conditions.