Ever wondered why this query fails in SQL? SELECT department_id , COUNT ( * ) AS emp_count FROM employees WHERE emp_count > 5 -- ❌ Error: Invalid column name 'emp_count' GROUP BY department_id ; The 6-Stage Execution Engine: FROM & JOIN — Load source tables & evaluate join conditions WHERE — Filter raw rows before grouping GROUP BY — Aggregate rows into buckets HAVING — Filter aggregated buckets ...