How can i do this in postgresql

Question

i have table like :
table : a
a_id, b_id, c_id, type, created_at
i should join a with table b or c
for each row, just one of the b_id or c_id is filled(according to type)
if b => a should join with b
c => a should join with c
how can i do this in postgresql

Rajanikant Hawaldar · Answer

You have to join all tables anyway. Just trust the DB, it will probably choose a proper strategy. Better than any we could come up with, most probably

Adarsh · Answer

yes!
but acctualy there is three foreign key
and always one of them is valid
and i always have 2 more join(performance issue? or because of primary key can not be null, there is no join for null foreign keys?)

Rajanikant Hawaldar · Answer

In that case you don't need that just classic LEFT JOINs

Adarsh · Answer

there is no condition like
"AND a = ..."
if type = b then b_id is valid and c_id is null
then i should join with b
and c is exactly like this

Rajanikant Hawaldar · Answer

Two left joins with "AND a = ..." in the join condition.