23. Sample 4GL Application : Employee Database : Referential Integrity
 
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Referential Integrity
The Employee_histories table is joined to the Employees table on "ssn" and to the Positions table on "position_code." The application ensures referential integrity; the user is not allowed to specify a Social Security number or position code in a record in the Employee_histories table if there is no corresponding record in the Employees table or Positions table.