Pre-determined join columns (PJCs) are another type of virtual column. Its value is looked up from a related table if the relationship is one to one. If the relationship is one to many, its value is calculated by retrieving records from a related table.
For example, this diagram describes the relationship between two tables:
•An Employee belongs to a Department
•A Department has many Employees
We could define a PJC for Employee, EmployeeDepartment which contains DepartmentDescription, looked up from the Departments table.
We could define a PJC, EmployeeSalaries for table Departments, which sums EmployeeSalary for all related Employee records.
There are two steps to defining a PJC:
•Define an access route which defines the relationship to the related table
•Define a pre-determined join column which uses this access route.