Anti-join finds rows in left table without matches in right table. Uses LEFT JOIN + WHERE right_table.primary_key IS NULL pattern.

LEFT JOIN preserves all left rows. WHERE IS NULL filters to only rows without matches. Check right table primary key (not foreign key) for NULL. Alternative: NOT EXISTS subquery.

Anti-joins are essential for gap analysis, data quality checks, and targeted campaigns. Every data analyst uses them to find missing relationships.

Find customers who never ordered (for marketing). Find products never sold (for inventory). Find employees without performance reviews (for HR). Anti-joins identify gaps in data.