A LEFT JOIN is unexpectedly returning the same results as INNER JOIN. Why?

The most common cause: a WHERE clause filtering on a column from the right table. Example: SELECT * FROM employees e LEFT JOIN departments d ON e.dept_id = d.dept_id WHERE d.dept_name = 'Engineering'. If dept_name is NULL (no match), the WHERE condition fails → that row is excluded. Solution: move the filter to the ON clause: LEFT JOIN departments d ON e.dept_id = d.dept_id AND d.dept_name = 'Engi