[T4] Query optimization patterns

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Module: sql-trainer/curriculum/04-expert/08-query-optimization.md
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Review checklist

• Technically accurate (SQL correct, claims true)
• Clear & at the right level for the tier
• Persona voice — plain English first, code only when it helps
• Examples run on the shared dataset and produce the described result
• Dialect notes correct (where present)
• Exercises solvable and match the concept taught

Definition of done

Reviewed and either (a) approved with no changes, or (b) refined together until the problem and solution align.