ER Diagrams   «Prev 

Illustrating many-to-many Relationships in a Database

Many to many relationships can only exist while you conceptually design the database and this is when you need to eliminate them. You physically cannot have it in a working database otherwise your schema would need to change continually.
A many-to-many (M:N) relationship, sometimes called non-specific, is when for one instance of entity A, there are zero, one, or many instances of entity B and for one instance of entity B there are zero, one, or many instances of entity A.
An example is:
"employees can be assigned to no more than two projects at the same time; projects must have assigned at least three employees"

The diagram above illustrates an M:N relationship between Table A and B from the viewpoint of Table A. Note that the record in Table A is related to many records in Table B (A:B = 1:N).

The relationship between Tables A and B from the viewpoint of Table B appears this way. Note that one record in Table B is related to many records in Table A (B:A = 1:M).