Database Concepts › Representing Relationships as Sets › Networks › Junction Records

Junction Records

A junction record is the common member record in a multiple membership structure. A junction record serves two functions:

• Enables the many-to-many relationship between its owners.
• Contains data specific to the intersection of the owner records.

The EMPOSITION record of the previous example enables the representation of the many-to-many relationship between the EMPLOYEE and JOB record types. At the same time, the contents of an EMPOSITION occurrence describe the relationship between its two owners by recording the beginning and ending dates of the employee's assignment to the project and their position on the project.

Another example of a many-to-many relationship at Commonweather is between employee and skills. An employee can have many skills while many employees can have the same skill. Since a direct EMPLOYEE-SKILL set is not possible, the EXPERTISE junction record is used to:

• Join the EMPLOYEE and SKILL record types in a many-to-many relationship
• Store an employee's proficiency in a skill

The following figure illustrates three occurrences of the SKILL-EXPERTISE set and the three related occurrences of the EMP-EXPERTISE set. The junction record (EXPERTISE) denotes a skill that an employee has.