Foreign key errors can also arise when performing update or delete operations. For example, suppose you try to remove the R&D department from the Departments table. The DepartmentID field, being the primary key of the Departments table, constitutes the ONE side of a one-to-many relationship (the DepartmentID field of the Employees table is the corresponding foreign key, and forms the MANY side). A record on the ONE side of a relationship may not be deleted until all corresponding records on the MANY side are deleted.
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