Question

I have two big tables:

A: (ID1,VAR1);
B: (ID1,ID2,VAR2)

And a third table:

C: (ID2...)

I’d like to merge A and B only for the records having values from C.

Below is my thought by using “IN”. Is there a more efficient way to do this? Sometimes I found that “IN” does not work so well.

Thanks a lot!

select  A.VAR1,B.VAR2
from A 
INNER JOIN 
(
  select ID1,VAR2
  from B
  where ID2 in
                (select distinct ID2
                 from C
                )
) D
on A.ID1=D.ID1

Answer 1

There is no need to use the subqueries, you just need to do a JOIN on all of the tables:

SELECT A.VAR1, B.VAR2
FROM A 
INNER JOIN B
    ON A.id1 = b.id1
INNER join
(
    SELECT DISTINCT id2
    FROM c
) c1
    ON b.id2 = c1.id2