Many-to-many (data model)

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

Data relationships

For example, think of A as Authors, and B as Books. An Author can write several Books, and a Book can be written by several Authors. In a relational database management system, such relationships are usually implemented by means of an associative table (also known as join table, junction table or 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 entity types represented by logical model database tables is sometimes referred to as a HasAndBelongsToMany (HABTM) relationship.

See also

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

References

1. One-to-many (data model)
2. Cardinality (data modeling)
3. Data modeling
4. Data model
5. One-to-one (data model)
6. Entity–relationship model
7. Common data model
8. SQL
9. OSI model
10. Associative entity
11. One-to-many
12. Data vault modeling
13. Relational model
14. Semantic data model
15. Database model
16. Predictive modelling
17. Foundation model
18. Data-driven model
19. Hierarchical database model
20. Training, validation, and test data sets