An associative entity is an element of the Entity-relationship model

Entity-relationship model

In software engineering, an entity-relationship model is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements...

. The database relational model

Relational model

The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by Edgar F...

 does not offer direct support to many-to-many relationships, even though such relationships happen frequently in normal usage. The solution to this problem is the creation of another table to hold the necessary information for this relationship. This new table is called an associative entity.

To create a relationship, a "child" entity must inherit the primary key of a "parent" entity. However, in a many-to-many relationship, neither entity is the "parent" or the "child"; the relationship is "unresolved". In order to work, these databases require an additional construct to "resolve" the relationship (which is why associative entities are also referred to as "resolving entities").

An associative entity can be thought of as both an entity and a relationship since it encapsulates properties from both. It is a relationship since it is serving to join two or more entities together, but it is also an entity since it may have its own properties. The associative entity must have identifiers, Primary Keys

Unique key

In relational database design, a unique key can uniquely identify each row in a table, and is closely related to the Superkey concept. A unique key comprises a single column or a set of columns. No two distinct rows in a table can have the same value in those columns if NULL values are not used...

, for both adjoining tables, but may also contain its own unique identifier and other information about the relationship.

The following guidelines may be used when considering the use of an associative entity:

• All relationships for the associative entity should be many.
• The associative entity could have meaning independent of the other entities.
• The associative entity preferably has a unique identifier, and should also have other attributes.
• The associative entity may participate in relationships other than the entities of the associated relationship.