Associative Entities and Many to Many Relationships in Databases

er model

Objects or things in the real world are represented in the database as entities, such as student, course, and teacher. When entities have complex relationships, then an associative entity is used, in order to minimize complexity. The occurrence of one or more entity types is associated together, in order to form an associative entity. The associative entity is often times used in a many-to-many relationship, where the relationship between the entities can be defined by another entity. The associative entity is also known as concatenated, junction, bridge or intersection entity. The main structure of an associative entity includes at least the primary keys of the related tables, as foreign keys, which are used to link the two entities, within the associative entity.

The primary key of the associative entity can be either a composite of the foreign keys, or it can be a whole new one. When entities are involved in a many to many relationships, they have the same right and left parent, and there are attributes, which are associated therefore creating a complex relationship between them; then associative entities can be used to allow the grouping of such entities. Primary keys are the identifiers of the relationship and make the relationship of the entities to be possible, in order for the many to many relationships to be modeled as an associative entity.

Database administrators (DBAs) can draw some important information from associative entities, regarding the entities’ attributes and relationship with each other. Associative entities, have attributes and link other entities together, which forces them to have many relationships, even outside of the entities the associative entity is linking together.

The logical relationships, between data items, must be recognized when a database needs to be designed; through data analysis, a database designer can recognize these relationships and achieve more, in order to properly design the database. By analyzing the relationships between entities and the process in which a database must work, negative activities, which could dominate the overall design process of the database, can be avoided. Understanding associative entities is part of Data model techniques, which can help DBAs produce data structures, which contain the most consistent data, which is not redundant. Data model techniques can be used to model the relationships, between two entities, and it is represented using diagram through a tool called the entity-relationship or ER Assistant, which produces ER diagrams. The diagram below depicts an example of an associative entity being used, in order to link the many to many relationships between, students and teachers the associative entity here is course, which associates or connects students and teachers, through their primary keys.


In today’s complex world, it can be very difficult to develop a good quality database; therefore, it is recommended to use a specific data model, in order to properly design a database. A DBA should expect for the model of a database to also evolve through a series of phases, where he/she would have to understand the application, the database will be used for. Once enough information about the application is gained, then the DBA would also obtain the input, which would be used to design the database and will be able to make important connections between data items. 

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