Does the Join Order Matter in Sql

Does the join order matter in SQL?

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.*.

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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

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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.

Does Sql JOIN order affect performance?

No, the JOIN by order is changed during optimization.

The only caveat is the Option FORCE ORDER which will force joins to happen in the exact order you have them specified.

Is the order of joining tables indifferent as long as we chose proper join types?

In an inner join, the ordering of the tables in the join doesn't matter - the same rows will make up the result set regardless of the order they are in the join statement.

In either a left or right outer join, the order DOES matter. In A left join B, your result set will contain one row for every record in table A, irrespective of whether there is a matching row in table B. If there are non matching rows, this is likely to be a different result set to B left join A.

In a full outer join, the order again doesn't matter - rows will be produced for each row in each joined table no matter what their order.

Regarding A left join B vs B right join A - these will produce the same results. In simple cases with 2 tables, swapping the tables and changing the direction of the outer join will result in the same result set.

This will also apply to 3 or more tables if all of the outer joins are in the same direction - A left join B left join C will give the same set of results as C right join B right join A.

If you start mixing left and right joins, then you will need to start being more careful. There will almost always be a way to make an equivalent query with re-ordered tables, but at that point sub-queries or bracketing off expressions might be the best way to clarify what you are doing.

As another commenter states, using whatever makes your purpose most clear is usually the best option. The ordering of the tables in your query should make little or no difference performance wise, as the query optimiser should work this out (although the only way to be sure of this would be to check the execution plans for each option with your own queries and data).

Does the order of tables in a join matter, when LEFT (outer) joins are used?

It is the same but it is ambiguous as hell with the implicit CROSS JOINs. Use explicit JOINS.

If you are joining in the WHERE clause then the results may differ because joins and filters are mixed up.

SELECT ....
  FROM apples a
       JOIN
       bananas b ON ...
       JOIN 
       oranges o ON ...
       LEFT JOIN
       kiwis k ON k.orange_id = o.id
 WHERE (filters only)

Notes:

• INNER JOINS and CROSS JOINS are commutative and associative: order does not matter usually.
• OUTER JOINS are not, which you identified
• SQL is declarative: you tell the optimiser what you want, not how to do it. This removes JOIN order considerations (subject to the previous 2 items)

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