Design a database schema for a library system that keeps track of books, patrons, and checkouts. The system should allow patrons to check out multiple books at a time, and should keep track of when books are due back.
Given a table with two columns, find the most common value in each column using a SQL query.
This technical problem deals with writing a query that returns the average salary of employees in each department. The given table includes information on the department, employee, and salary. The expected output is a table that lists the department and the average salary for that department.