Can I specify the order in which the joins occur? - mysql

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!

Answer 2

Edit: And that's what I get for talking faster than I think. My previous solution doesn't work because 'c' hasn't been joined yet. Let's try this again.
We can use a WHERE clause to limit the results to only those that match the criteria you're looking for, where C has a valid (IS NOT NULL) or B does not have a value (IS NULL). Like this:
SELECT *
FROM a
LEFT JOIN b ON (b.a = a.a)
LEFT JOIN c ON (c.b = b.b)
WHERE (c.c IS NOT NULL OR b.b IS NULL);
Without WHERE Results:
mysql> SELECT * FROM a LEFT JOIN b ON (b.a = a.a) LEFT JOIN c ON (c.b = b.b);
+------+------+------+------+------+
| a | a | b | c | b |
+------+------+------+------+------+
| 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 2 | NULL | NULL |
| 2 | 2 | 3 | 2 | 3 |
| 3 | NULL | NULL | NULL | NULL |
| 4 | NULL | NULL | NULL | NULL |
+------+------+------+------+------+
With WHERE Results:
mysql> SELECT * FROM a LEFT JOIN b ON (b.a = a.a) LEFT JOIN c ON (c.b = b.b) WHERE (c.c IS NOT NULL OR b.b IS NULL);
+------+------+------+------+------+
| a | a | b | c | b |
+------+------+------+------+------+
| 1 | 1 | 1 | 1 | 1 |
| 2 | 2 | 3 | 2 | 3 |
| 3 | NULL | NULL | NULL | NULL |
| 4 | NULL | NULL | NULL | NULL |
+------+------+------+------+------+

Answer 3

Yes, you use the STRAIGHT_JOIN for this.
When using this keyword the join will occur in the exact order that you specify.
See: http://dev.mysql.com/doc/refman/5.5/en/join.html

Answer 4

Well, I thought up another solution as well, and I'm posting it for completeness (Though I'm actually using Martin's answer).
Use a RIGHT JOIN:
SELECT
*
FROM
b
INNER JOIN c ON b.c_fk_id = c.c_id
RIGHT JOIN a ON a.id = b.a_fk_id
I'm pretty sure every piece I've read about JOINS said that RIGHT JOINs were pointless, but there you are.