Does Join Order Affect SQL Query Results, Especially with Mixed Inner and Outer Joins?

SQL Join Order: Does It Impact Results?

This article explores whether altering the join order in SQL queries affects the outcome, particularly when combining inner and outer joins. We'll disregard performance implications for this analysis.

Inner Joins:

The order of tables in inner joins is inconsequential. The results remain identical, provided the SELECT clause is adjusted to explicitly list columns (e.g., SELECT a.*, b.*, c.* instead of SELECT *).

Outer Joins (LEFT, RIGHT, FULL):

The situation differs significantly for outer joins:

• Non-Commutativity: Outer joins are not commutative. a LEFT JOIN b is not the same as b LEFT JOIN a.
• Non-Associativity: Outer joins are also not associative. Consider this example:

<code class="language-sql">a LEFT JOIN b ON b.ab_id = a.ab_id
LEFT JOIN c ON c.ac_id = a.ac_id</code>

• Equivalence (with caveats): This query is equivalent to:

<code class="language-sql">a LEFT JOIN c ON c.ac_id = a.ac_id
LEFT JOIN b ON b.ab_id = a.ab_id</code>

• Non-Equivalence (important distinction): However, this is not equivalent:

<code class="language-sql">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</code>

Join Condition Structure:

The structure of the join conditions is crucial. For instance:

<code class="language-sql">a LEFT JOIN b ON b.ab_id = a.ab_id
LEFT JOIN c ON c.bc_id = b.bc_id</code>

is only equivalent to:

<code class="language-sql">a LEFT JOIN (b LEFT JOIN c ON c.bc_id = b.bc_id) ON b.ab_id = a.ab_id</code>

under the condition that the join conditions utilize only equality checks without NULL value comparisons. The inclusion of NULL comparisons or functions like COALESCE() can break the equivalence between differently ordered queries.

In summary, while inner join order doesn't matter, outer join order significantly impacts results. Careful consideration of join conditions, particularly concerning NULL values, is essential when working with outer joins.

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