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The Entity-Relationship model represents data abstractly in software engineering. Associative entities connect two different entities and can have many-to-many relationships. Primary keys are important for identifying tables in a relational model, and associative entities are contained in junction tables to handle many-to-many relationships.
In the Entity-Relationship model, where data is represented abstractly and conceptually in software engineering, associative entities are a part of the model. Relational models are often built to represent data in a database or system, where they are used to organize the data into an understandable and readable schema. Entities in a relational model could represent anything, including people, places, objects, events, or concepts. Associative entities are connections that describe a relationship between two different entities. These entities can have many-to-many relationships, which means that one of the associative entities can have multiple relationships and connections to a parent or child entity.
Associative entities convey information about their attributes and their connections. They are considered an entity because they have attributes and are considered a relationship because they connect entities to each other. These types of entities very frequently have many relationships and connections because they are attributable, but may have independent meaning from other entities. It is also good practice in a relational model that the associative entity has at least one attribute that is distinguishable from the identifying entity. Associative entities can also be participants in relationships separate from associated entity relationships.
Primary keys should also be part of associative entities, because they are identifiers that abut certain tables in a relational model. Although tables can contain only one, primary keys are combinations of columns that uniquely specify rows. There is a difference between unique keys and primary keys, in that primary keys can impose a “non-null” constraint on a table or entity. Another aspect that differentiates primary keys from unique keys is that primary keys are selected as the most or foremost key. Primary indexes are created for primary key management and for ease of use and enforcement by a database manager.
Associative entities are contained and used by junction tables: tables in a relational model that contain common fields from two or more other tables. Junction tables are used because they can handle many-to-many relationships in a particular database. An example of a database that uses the associative entity through the use of the junction table is the enrollment of students in a course. In this case a table with student data is indirectly linked to a table with course data via an intermediate table that contains the student data and the course data from the linking tables.
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