I have a relation between two tables. The foreign key table can have a row related in the primary table. There's a way to enforce the value of the FK column to be NULL or one of the values of the PK column in the primary table?
Yes, you can allow a foreign key column to be NULL, making it an optional relation.
CREATE TABLE dbo.foo(fooid INT PRIMARY KEY);
CREATE TABLE dbo.bar(barid INT PRIMARY KEY,
fooid INT NULL FOREIGN KEY REFERENCES dbo.foo(fooid));
INSERT dbo.foo SELECT 1;
INSERT dbo.bar SELECT 1,1;
INSERT dbo.bar SELECT 2,NULL; -- succeeds
INSERT dbo.bar SELECT 3,2; -- fails
If this is not what you meant, then please elaborate or re-word your question.