Most Public Interface views contain a column that is a foreign key. Foreign keys enable users to combine views by using joins. When combined, the views can be referenced as if they were a single view.
For example, one of the columns of the view ATTRIBUTE is a foreign key named Parent_Entity_ID. This column can be joined, or matched, with the ID column of the view ENTITY_TYPE to determine the parent entity type for the attribute. Joining uses the unique encyclopedia identifier- 7953-which the attribute's parent entity type and the entity type share.
Joining the foreign key column (Parent_Entity_ID) with the target column (ID in the view ENTITY_TYPE) shows that the attribute Governor belongs to the entity type State. You use the join technique illustrated by the previous figure and described later in this section to select objects from the Public Interface.
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