Multirelational data mining aims to discover knowledge directly from relational data. There are different multirelational data mining tasks, including multirelational classi- fication, clustering, and frequent pattern mining. Multirelational classification aims to build a classification model that utilizes information in different relations. Multirela- tional clustering aims to group tuples into clusters using their own attributes as well as tuples related to them in different relations. Multirelational frequent pattern mining aims at finding patterns involving interconnected items in different relations. We first use multirelational classification as an example to illustrate the purpose and procedure of multirelational data mining. We then introduce multirelational classification and mul- tirelational clustering in detail in the following sections.
In a database for multirelational classification, there is one target relation, Rt , whose tuples are called target tuples and are associated with class labels. The other relations are nontarget relations. Each relation may have one primary key (which uniquely identifies tuples in the relation) and several foreign keys (where a primary key in one relation can
be linked to the foreign key in another). If we assume a two-class problem, then we pick one class as the positive class and the other as the negative class. The most important task for building an accurate multirelational classifier is to find relevant features in different relations that help distinguish positive and negative target tuples.