Assume two entities, A and B.
A M:M relationship exists between entity A and entity B when each occurrence of entity A is related 0, 1, or many occurrences of entity B and each occurrence of entity B is related to 0, 1, or many occurrences of entity A.
M:M relationships are commonly seen in real world situations.
M:M relationships are graphed as follows:
Modeling M:M Relationships
M:M relationships are modeled by placing the primary key of entity A and the primary key of entity B as foreign keys comprising the entire primary key of a separate entity referred to as an associative table. This is another example of the subjectivity of E-R theory, because a relationship is expressed as a physical table in the database.
Guideline for Drawing M:M Tables for Users
Draw entities that represent M:M relationships in a way that data is presented in a form most familiar to users.
The following set of entities models the M:M relationship between A and B: