If I understand the question correctly, you define a foreign key when creating the child table. For example
CREATE TABLE Parent (
ParentId int NOT NULL PRIMARY KEY,
SomeData varchar(100) NOT NULL
);
CREATE TABLE Child (
ChildId int NOT NULL PRIMARY KEY,
ParentId int NOT NULL FOREIGN KEY REFERENCES Parent (ParentId),
OtherData varchar(100) NOT NULL
);
...
For more information, see
Create Foreign Key Relationships[
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