Question

Say I have three tables, A, B and C. Conceptually A (optionally) has one B, and B (always) has one C.

Table A:
a_id
... other stuff

Table B:
a_fk_id (foreign key to A.a_id, unique, primary, not null)
c_fk_id (foreign key to C.c_id, not null)
... other stuff

Table C:
c_id
... other stuff

I want to select All records from A as well as their associated records from B and C if present. However, the B and C data must only occur in the result if both B and C are present.

I feel like I want to do:

SELECT *
FROM
    A
    LEFT JOIN B on A.a_id=B.a_fk_id
    INNER JOIN C on B.c_fk_id=C.c_id

But Joins seem to be left associative (the first join happens before the second join), so this will not give records from A that don’t have an entry in C.

AFAICT I must use sub queries, something along the lines of:

SELECT *
FROM
    A
    LEFT JOIN (
        SELECT * FROM B INNER JOIN C ON B.c_fk_id=C.c_id
    ) as tmp ON A.id = tmp.a_fk_id

but once I have a couple of such relationships in a query (in reality I may have two or three nested), I’m worried both about code complexity and about the query optimizer.

Is there a way for me to specify the join order, other than this subquery method?

Thanks.

Answer 1

In SQL Server you can do

SELECT *
FROM   a
       LEFT JOIN b
                 INNER JOIN c
                   ON b.c_fk_id = c.c_id
         ON a.id = b.a_fk_id  

The position of the ON clause means that the LEFT JOIN on b logically happens last. As far as I know this is standard (claimed to be ANSI prescribed here) but I’m sure the downvotes will notify me if it doesn’t work in MySQL!