当插入引用其他两个表的第三个表时，如何检查两个表之间的约束？

Consider this example schema:

考虑这个示例模式:

Customer ( int CustomerId pk, .... )

Employee ( int EmployeeId pk,
           int CustomerId references Customer.CustomerId, .... )

WorkItem ( int WorkItemId pk,
           int CustomerId references Customer.CustomerId,
           null int EmployeeId references Employee.EmployeeId, .... )
Cus