N-ary Relation (ERD Styles)

• N-ary relations are many-to-many associations among N object instances (of the same or different types).

• N-way associations can be implemented by introducing a new associative entity, whose tuples contain exactly N fkeys (plus optional non-key attributes). (Most relations are binary: N = 2).

• The diamond indicats a ternary relation among types AA, BB and CC. [It is superfluous if N=2, if the relation is one to many, or if an associative entity replaces it.]

Optional

attributes

AA

BB

CC

AA

BB

CC

AABBCC

New Entity AABBCC gives

these atributes a home, and replaces the diamond.

(3 fkeys

inside)

Example

for N=3:

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