SQL Server 2014

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CREATE TABLE Test
(
    Id1 INT NOT NULL,
    Id2 INT NOT NULL,
    PRIMARY KEY (Id1, Id2)
)

CREATE TABLE Test2
(
    TestId1 INT NOT NULL,
    TestId2 INT NULL,
    UNIQUE CLUSTERED (TestId1, TestId2),
    FOREIGN KEY (TestId1, TestId2) REFERENCES Test (Id1, Id2)
)

CREATE TABLE Test3
(
    TestId1 INT NOT NULL,
    TestId2Calc AS COALESCE(TestId2, -1) PERSISTED NOT NULL,
    TestId2 INT NULL,
    PRIMARY KEY (TestId1, TestId2Calc),
    FOREIGN KEY (TestId1, TestId2) REFERENCES Test (Id1, Id2)
)
-- Should work
INSERT INTO Test VALUES (1, 1)
INSERT INTO Test2 VALUES (1, NULL)
INSERT INTO Test3 VALUES (1, NULL)
INSERT INTO Test2 VALUES (1, 1)
INSERT INTO Test3 VALUES (1, 1)
INSERT INTO Test2 VALUES (2, NULL)
INSERT INTO Test3 VALUES (2, NULL)
7 rows affected

-- Should error
INSERT INTO Test2 VALUES (2, 1)
INSERT INTO Test3 VALUES (2, 1)
Msg 547 Level 16 State 0 Line 2
The INSERT statement conflicted with the FOREIGN KEY constraint "FK__Test2__1273C1CD". The conflict occurred in database "fiddle_70fd0125e1704679b1626b8dadfca216", table "dbo.Test".
Msg 547 Level 16 State 0 Line 3
The INSERT statement conflicted with the FOREIGN KEY constraint "FK__Test3__15502E78". The conflict occurred in database "fiddle_70fd0125e1704679b1626b8dadfca216", table "dbo.Test".
Msg 3621 Level 0 State 0 Line 2
The statement has been terminated.
Msg 3621 Level 0 State 0 Line 3
The statement has been terminated.

SELECT * FROM Test
SELECT * FROM Test2
SELECT * FROM Test3


Id1
Id2


1
1




TestId1
TestId2


1
null

1
1

2
null




TestId1
TestId2Calc
TestId2


1
-1
null

1
1
1

2
-1
null


DROP TABLE Test3
DROP TABLE Test2
DROP TABLE Test
TestId1	TestId2
1	null
1	1
2	null