Introducing complexity through join tables

Let's take the model, which was described previously, to the database administrator for an actual implementation. What happens then is that, in this implementation, the relational model inherently causes complexity. What you can see in the following diagram is that, for every relationship where we can have n-n combinations, we actually need to introduce something that links the two tables together.

This is what we call a join table, and this will be used by every query that requests a combination of the n-n entities:

In the previous example, we introduced the AppDatabase table to link applications to database servers and the UserApp table to link Users to Applications. These join tables are only necessary for dealing with the shortcomings of the relational model, and they complicate our lives as database administrators and application developers. They introduce unwanted complexity.