Why does a multicolumn foreign key allow nonexisting values [duplicate] - sql-server

Question

This question already has answers here:
Foreign Key Constraint in child table allows to insert values that doesn't exists in the parent table
(2 answers)
Closed 3 years ago.
Consider the following tables:
CREATE TABLE Foo
(
FooId1 int NOT NULL,
FooId2 int NOT NULL,
PRIMARY KEY(FooId1, FooId2)
);
INSERT INTO Foo(FooId1, FooId2) VALUES (1, 10), (2, 20);
CREATE TABLE Bar
(
BarId int NOT NULL PRIMARY KEY,
FooId1 int NULL,
FooId2 int NULL
);
ALTER TABLE Bar ADD CONSTRAINT FK_FOO
FOREIGN KEY(FooId1, FooId2)
REFERENCES Foo(FooId1, FooId2);
INSERT INTO Bar(BarId) VALUES(42);
If I then try
UPDATE Bar
SET FooId1 = 3,
FooId2 = 30
WHERE BarId = 42;
I'm getting an foreign key violation as expected. But if I do
UPDATE Bar
SET FooId1 = NULL,
FooId2 = 30
WHERE BarId = 42;
it works without an error. I would have expected that even though the fields FooId1 and FooId2 are nullable the combination of both would always be checked against the foreign key. Why does the DB allow this UPDATE?

Answer 1

When a value other than NULL is entered into the column of a FOREIGN
KEY constraint, the value must exist in the referenced column;
otherwise, a foreign key violation error message is returned. To make
sure that all values of a composite foreign key constraint are
verified, specify NOT NULL on all the participating columns.
Create Foreign Key Relationships
According to this explanation of the MSDN, you should define the Food1 and Food2 columns of the Bar columns with NOT NULL constraint.
CREATE TABLE Bar
(
BarId int NOT NULL PRIMARY KEY,
FooId1 int NOT NULL,
FooId2 int NOT NULL
)
So that the following insert will not work;
INSERT INTO Bar(BarId) VALUES(42)
Msg 515, Level 16, State 2, Line 23
Cannot insert the value NULL into column 'FooId1', table 'AdventureWorks2017.dbo.Bar'; column does not allow nulls. INSERT fails.
The statement has been terminated.