How do you write a SQL query to find departments that have no employees using a LEFT JOIN?

To find departments without employees, perform a LEFT JOIN from the department table to the employee table and filter rows where the employee key is NULL. Example:

SELECT d.deptno, d.dname

FROM dept d

LEFT JOIN emp e ON d.deptno = e.deptno

WHERE e.empno IS NULL;

The LEFT JOIN keeps all departments, attaching employee rows when they exist. When a department has no matching employee, the joined columns are NULL, so the WHERE clause selects those rows. This query runs in O(n + m) time, where n is the number of departments and m the number of employees, assuming appropriate indexes on deptno. It is a standard pattern for identifying orphaned or missing related records.