What is a self join?

SELECT e.name AS employee, m.name AS manager FROM employees e LEFT JOIN employees m ON e.manager_id = m.emp_id;

What does this query find?

SELECT e.name, e.salary, m.name, m.salary FROM employees e JOIN employees m ON e.manager_id = m.emp_id WHERE e.salary > m.salary;

What does this find?

SELECT a.name AS emp1, b.name AS emp2, a.dept_id FROM employees a JOIN employees b ON a.dept_id = b.dept_id AND a.emp_id < b.emp_id;

What does a.emp_id < b.emp_id prevent?

SELECT p1.name, p2.name, p1.price FROM products p1 JOIN products p2 ON p1.price = p2.price AND p1.id < p2.id;

What does this query find?