<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2013) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 instance for which there are many children in B and vice versa . </P> <P> 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 . </P> <P> In a relational database management system, such relationships are usually implemented by means of an associative table (also known as 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). </P>

When must a many-to-many relationship be modelled as an associative entity