Subqueries vs Joins

Subqueries vs Joins

Joins and subqueries often solve identical data retrieval problems, but their internal execution plans differ.

When to Use Joins

• When retrieving columns from multiple tables in the final output.
• For large datasets where the optimizer can utilize Hash Joins or Merge Joins.
• When joining on indexed primary and foreign keys.

When to Use Subqueries

• Checking set membership using IN or existence using EXISTS.
• Comparing individual rows against aggregated benchmarks:
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  SELECT * FROM orders WHERE total > (SELECT AVG(total) FROM orders);

• When isolating modular calculations before feeding into the outer query.