Microsoft SQL Server Foreign Keys Adding foreign key relationship on existing table
Example
FOREIGN KEY constraint can be added on existing tables that are still not in relationship. Imagine that we have Company and Employee tables where Employee table CompanyId column but don't have foreign key relationship. ALTER TABLE statement enables you to add foreign key constraint on an existing column that references some other table and primary key in that table:
alter table Employee
add foreign key (CompanyId) references Company(CompanyId)
