外键可以是可选的吗

Question

Table employee
{
employeename  //employeename is unique
}

Table job
{
jobid,job,assignedPerson,status
}

assignedPerson is a foreign key that references to employeename(employee)
What I want is to know how to delete an employee record without having to remove his record from the job table ie I mean I want to keep the job record and see which person was assigned the job even if that person is no longer an employee..

Table employee
{
employeename
}

Table jobemployee
{
jobid,assignedPerson
}

Table job
{
jobid,job,status
}

Even if I do this I will have to delete the record from jobemployee if I want to delete the record of employee from 'employee' table and will not be able to know which person was assigned the job..
Please help..

Answer 1

The best option is to add a flag to the Employee table for deleted .

A workaround is to add a column assignedNonEmployee to the job table. Before you delete an employee , you set the assignedPerson column to null , and copy its value into the assignedNonEmployee column. Since the later column has no foreign key constraint, you are now allowed to delete the employee record.