Now that you know how to write simple queries using one table and how to use functions and expressions in queries, it is time to learn how to write queries involving two or more tables. In a properly designed relational database, a table contains data about one thing or entity. For example, an order-entry application will have a table storing customer information, a table containing data about orders, and a table containing detail information about each item ordered. The order table has a column, called a
, that points to a row in the customer table. The detail table has a foreign key column that points to the order table. By using
, you can link these tables together so that you can display columns from each table in the same result set.