Problem

Given a table with the following schema: 
CREATE TABLE Employee (
    ID int, 
    Name varchar(255), 
    ManagerID int, 
    Department varchar(255)
);
Write a SQL query that returns the ID and name of all employees who are not managers and who are in the Sales department. 
Example input: 
ID | Name  | ManagerID | Department
--------------------------------------
1  | John  | 3         | Sales
2  | Jane  | 3         | Marketing
3  | Joe   | NULL      | Sales
4  | Karen | 2         | Sales
5  | Carl  | 2         | Marketing
Example output: 
ID | Name
---------
3  | Joe
4  | Karen

Solution

SELECT ID, Name
FROM Employee
WHERE ManagerID IS NULL AND Department = 'Sales';

/*

The solution is optimal because it uses the WHERE clause to filter out the rows that do not meet the criteria. 
The WHERE clause is evaluated before the SELECT clause, so the query will only return the rows that meet the criteria. 
This is optimal because it will not return any rows that do not meet the criteria, and it will not return any rows that do meet the criteria more than once.

*/