SQL : matching two tables with two possibles conditions - sql

Question

is there a way in SQL while we join two tables table_A and table_B, if we can’t match the two tables on a criteria said criteria_X at all we will try the second criteria criteria_Y
Something like this:
select *
from table_A, table_B
where table_A.id = table_B.id2
and (if there is no row where table_B.criteria_X = X then try table_B.criteria_Y = Y)
The following query is not a solution:
..
and (table_B.criteria_X = X OR table_B.criteria_Y = Y)
Thanks

Answer 1

This is a find the best match query:
select *
from
(
select *,
row_number() -- based on priority, #1 criteria_X, #2 criteria_Y
over (partition by table_A.id
order by case when table_B.criteria_X = X then 1
else 2
end) as best_match
from table_A, table_B
where table_A.id = table_B.id2
and (table_B.criteria_X = X OR table_B.criteria_Y = Y)
) dt
where best_match = 1
If the ORed condition results in loosing indexed access you might try splitting it into two UNION ALL selects.

Answer 2

A typical method uses left join twice . . . once for each criterion. Then then uses coalesce() in the select. And, with indexes on the join keys, this also should have very good performance:
select a.*, coalesce(b1.colx, b2.colx)
from table_A a left join
table_B b1
on a.id = b1.id2 and b1.criteria_X = X left join
table_B b2
on a.id = b1.id2 and b2.criteria_Y = Y
where b1.id2 is not null or b2.id2 is not null;
The where clause ensures that at least one row matches.
This does not work under all circumstances -- in particular, each join needs to return only 0 or 1 matching rows. This is often the situation with this type of "priority" joins.
An alternative version uses row_number(). This is sort of similar to #dnoeth's approach, but the row number calculation is done before the join:
select a.*, coalesce(b1.colx, b2.colx)
from table_A a join
(select b.*,
row_number() over (partition by id2
order by (case when criteria_x = X then 1
when criteria_y = Y then 2
end)
) as seqnum
from table_B b
where criteria_x = X or criteria_y = Y
) b
on a.id = b.id2 and seqnum = 1