When application developers use PHP and MySQL to build web interfaces or other data manipulation applications, they usually establish relations between tables using the underlying SQL queries. Examples of this would be: "get an invoice and all its items" and "get all books by an author".

In the first versions of phpMyAdmin, MySQL stored information about which table belonged to which database. However, the relational data structure (how tables relate to each other) was not stored into MySQL. Relations were temporarily made by the applications to generate meaningful results. In other words, the relations were in our head.