When you create a relationship between 2 table, a master and a slave table. You use a foreign key to be used as a field to create the relationship on.
In your code (from your comment) it is clear that the ID that integer, would be a FOREIGN KEY. As
Microsoft SQL Server documentation[
^] says,
Quote:
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables. You can create a foreign key by defining a FOREIGN KEY constraint when you create or modify a table.
Read the attached link, it has more information on this chapter and you will find more details to it there.
Thanks.