Does the join order matter in SQL? - sql

Question

Disregarding performance, will I get the same result from query A and B below? How about C and D?
----- Scenario 1:
-- A (left join)
select *
from a left join b
on <blahblah>
left join c
on <blahblan>
-- B (left join)
select *
from a left join c
on <blahblah>
left join b
on <blahblan>
----- Scenario 2:
-- C (inner join)
select *
from a join b
on <blahblah>
join c
on <blahblan>
-- D (inner join)
select *
from a join c
on <blahblah>
join b
on <blahblan>

Answer 1

For INNER joins, no, the order doesn't matter. The queries will return same results, as long as you change your selects from SELECT * to SELECT a.*, b.*, c.*.
For (LEFT, RIGHT or FULL) OUTER joins, yes, the order matters - and (updated) things are much more complicated.
First, outer joins are not commutative, so a LEFT JOIN b is not the same as b LEFT JOIN a
Outer joins are not associative either, so in your examples which involve both (commutativity and associativity) properties:
a LEFT JOIN b
ON b.ab_id = a.ab_id
LEFT JOIN c
ON c.ac_id = a.ac_id
is equivalent to:
a LEFT JOIN c
ON c.ac_id = a.ac_id
LEFT JOIN b
ON b.ab_id = a.ab_id
but:
a LEFT JOIN b
ON b.ab_id = a.ab_id
LEFT JOIN c
ON c.ac_id = a.ac_id
AND c.bc_id = b.bc_id
is not equivalent to:
a LEFT JOIN c
ON c.ac_id = a.ac_id
LEFT JOIN b
ON b.ab_id = a.ab_id
AND b.bc_id = c.bc_id
Another (hopefully simpler) associativity example. Think of this as (a LEFT JOIN b) LEFT JOIN c:
a LEFT JOIN b
ON b.ab_id = a.ab_id -- AB condition
LEFT JOIN c
ON c.bc_id = b.bc_id -- BC condition
This is equivalent to a LEFT JOIN (b LEFT JOIN c):
a LEFT JOIN
b LEFT JOIN c
ON c.bc_id = b.bc_id -- BC condition
ON b.ab_id = a.ab_id -- AB condition
only because we have "nice" ON conditions. Both ON b.ab_id = a.ab_id and c.bc_id = b.bc_id are equality checks and do not involve NULL comparisons.
You can even have conditions with other operators or more complex ones like: ON a.x <= b.x or ON a.x = 7 or ON a.x LIKE b.x or ON (a.x, a.y) = (b.x, b.y) and the two queries would still be equivalent.
If however, any of these involved IS NULL or a function that is related to nulls like COALESCE(), for example if the condition was b.ab_id IS NULL, then the two queries would not be equivalent.

Answer 2

If you try joining C on a field from B before joining B, i.e.:
SELECT A.x,
A.y,
A.z
FROM A
INNER JOIN C
on B.x = C.x
INNER JOIN B
on A.x = B.x
your query will fail, so in this case the order matters.

Answer 3

for regular Joins, it doesn't. TableA join TableB will produce the same execution plan as TableB join TableA (so your C and D examples would be the same)
for left and right joins it does. TableA left Join TableB is different than TableB left Join TableA, BUT its the same than TableB right Join TableA

Answer 4

Oracle optimizer chooses join order of tables for inner join.
Optimizer chooses the join order of tables only in simple FROM clauses .
U can check the oracle documentation in their website.
And for the left, right outer join the most voted answer is right.
The optimizer chooses the optimal join order as well as the optimal index for each table. The join order can affect which index is the best choice. The optimizer can choose an index as the access path for a table if it is the inner table, but not if it is the outer table (and there are no further qualifications).
The optimizer chooses the join order of tables only in simple FROM clauses. Most joins using the JOIN keyword are flattened into simple joins, so the optimizer chooses their join order.
The optimizer does not choose the join order for outer joins; it uses the order specified in the statement.
When selecting a join order, the optimizer takes into account:
The size of each table
The indexes available on each table
Whether an index on a table is useful in a particular join order
The number of rows and pages to be scanned for each table in each join order