A constraint-based knowledge-based recommendation engine is simply a user interface to a database backend. This is a step up from the filtering example because it allows potentially millions and millions of records (recipes in this case) to be queried. Advanced queries can be used with any combination to create the SQL that the database processes and then returns a result.

An example query could be to include "duck" and exclude "sugar."

In this particular example there could be many improvements made. For example, the recommendation engine could allow many ingredients to be included or excluded in the query.

DISCLAIMER: This is an example recommendation engine for the purposes of learning and teaching. There are many other features that could be added.