SQL Server WHERE Clause and Filtering: NULL Handling

Filter Employees with NULL ManagerID

Write a query to select all employees whose ManagerID is NULL.

Expected output: List of employees without a manager.

Hint: Use the IS NULL operator in the WHERE clause.

Filter Employees with Assigned Managers

Write a query to select all employees whose ManagerID is not NULL.

Expected output: List of employees with a manager assigned.

Hint: Use the IS NOT NULL operator in the WHERE clause.

Identify Incorrect NULL Comparison

Explain why the query `SELECT * FROM Employees WHERE ManagerID = NULL;` returns no rows.

Expected output: Explanation that = NULL returns UNKNOWN and no rows match.

Hint: Consider how SQL evaluates NULL comparisons.

You filter NULL values using the IS NULL or IS NOT NULL operators in the WHERE clause.

Because NULL represents unknown data, comparisons with = return UNKNOWN, so no rows match. IS NULL must be used instead.

NULL means no data or unknown, while an empty string is a known value with zero length.