The NOT EXISTS operator in SQL is used to check if a subquery returns any rows. It returns TRUE if the subquery does not return any rows, and FALSE if the subquery does return one or more rows.

• SELECT 1 is commonly used in EXISTS and NOT EXISTS subqueries to simplify and optimize the query.
• the focus is not on the data returned by the subquery but on whether any rows exist.

SELECT e.Name
FROM Employees e
WHERE NOT EXISTS (
    SELECT 1
    FROM Orders o
    WHERE o.EmployeeID = e.EmployeeID
);