What are window functions?

SELECT name, department, salary, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) FROM employees;

What does PARTITION BY do?

What's the difference between RANK() and DENSE_RANK()?

SELECT order_date, amount, SUM(amount) OVER (ORDER BY order_date) FROM orders;

What does this calculate?

SELECT name, salary, LAG(salary, 1) OVER (ORDER BY salary) FROM employees;

What does LAG() do?