Many-to-many (data model)

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An Author can write several Books, and a Book can be written by several Authors

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The Author-Book many-to-many relationship as a pair of one-to-many relationships with a junction table

In systems analysis, a many-to-many relationship is a type of cardinality that refers to the relationship between two entities A and B in which A may contain a parent record for which there are many children in B and vice versa.

For instance, think of A as Authors, and B as Books. An Author can write several Books, and a Book can be written by several Authors.

Because most database management systems only support one-to-many or one-to-one relationships, it is necessary to implement such relationships physically via a third junction table (also called cross-reference table), say, AB with two one-to-many relationships A -> AB and B -> AB. In this case the logical primary key for AB is formed from the two foreign keys (i.e. copies of the primary keys of A and B).

In web application frameworks such as CakePHP and Ruby on Rails, a many-to-many relationship between database tables in a model is sometimes referred to as a HasAndBelongsToMany (HABTM) relationship.

See also

• Associative Entities
• One-to-one (data model)
• One-to-many (data model)

External links

• Design pattern: many-to-many (order entry), Tomjewett.com