Many-to-Many Relationships in the Logical Model

A many-to-many relationship is a relationship between two entities where instances in one entity have zero, one, or more related instances in the other entity. In IDEF1X notation, a many-to-many relationship is drawn as a solid line with a solid dot on both the child end and the parent end. In IE notation, a many-to-many relationship is drawn as a solid line with crows feet on both the child end and the parent end.

If you choose to include many-to-many relationships in a logical model, you may encounter some design problems. To avoid design problems that may be caused by many-to-many relationships, database designers frequently add associative entities to act as links for two ambiguously related entities. If you insert an associative entity between the two entities involved in a many-to-many relationship, you can change the relationship into a series of one-to-many relationships and use the new relationships to clarify how the data model works.

You can automatically resolve many-to-many relationships. When a many-to-many relationship is resolved, an associative entity is displayed in the logical model and also eliminates the many-to-many relationship. You can resolve the relationship when you right-click a many-to-many relationship and click Create Association Entity, or by clicking the Many to Many Transformation icon on the Transformations toolbar.

When you select DM (Dimensional Modeling) notation for your physical model, the associative table is a fact table.