Does Join Order Affect SQL Query Results?

SQL Join Order: Does it Impact Results?

Outer Joins: Order Matters

The order of operations significantly impacts the results of LEFT, RIGHT, and FULL OUTER JOINs.

Commutativity and Associativity: Not Guaranteed

• Outer joins are not commutative. A LEFT JOIN B is different from B LEFT JOIN A.
• Outer joins are not associative. The order in which multiple joins are performed directly affects the outcome. Therefore:

<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>

is not guaranteed to be the same as:

<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>

Scenarios Where Order is Critical

Join order becomes crucial when dealing with AND conditions involving NULL values or functions sensitive to NULLs (like COALESCE()). For instance:

<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>

This query will yield a different result than:

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

Inner Joins: Order Independence

Unlike outer joins, the order of INNER JOINs does not affect the final result set. A JOIN B and B JOIN A produce identical results (assuming appropriate adjustments to the SELECT clause, e.g., SELECT a.*, b.*, c.*).

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