As a deliberate choice, Database Design Studio does not have a ternary or N-ary relationship type. The lack of standardization for translating a ternary or N-ary relationship to a logical model, has led us to adopt the process of reducing N-ary relationships to their equivalent binary relationships. The following describes how to reduce a ternary relationship to its corresponding binary conceptual model.
The relationship "attendance" is a ternary relationship between the three entities, Student, Subject and Room. Similar to a composite entity, it needs to be reduced to a table in the logical model, with its primary key made up of the primary keys from the three entities to which it is related, as well as an attendance date and time pair of keys.
Using Database Design Studio's weak entity and weak relationship types, the conceptual model can be created as follows:
Notice that Attendance is a weak entity type, and the three relationships are also weak. Together with the appropriate Connectivity of 1:M ensures that the primary keys of the dominant entities become foreign primary keys in the weak entity, as the corresponding Data Structure Diagram reveals:
Notice that we have added the primary keys, attDate and attTime to the Attendance weak entity in the Entity Relationship Diagram editor, and used the Data Structure Diagram editor to order the columns in the logical object.
See also: Foreign Primary Keys, Renaming Foreign Key Columns and Adding Keys to a Relationship