Let's say I want to select max attribute between two columns on a query. it isn't the whole table max.
Something like that:
SELECT MAX(t1.c1, t2.c2)
FROM Table1 as t1, Table2 as t2
WHERE t1.id = t2.t1_id
AND ...... here goes more conditions.
select max(c1) from (select c1 from t1 union all select c2 from t2) t;
Or -- probably more efficient -- use
select greatest(c1, c2) from (select max(c1) as c1 from t1) t1 cross join (select max(c2) as c2 from t2) t2;