Cannot create the foreign key “%.*ls” with the SET NULL referential action, because one or more referencing columns are not nullable.

Error Message:
Msg 1761, Level 16, State 0, Line 3
Cannot create the foreign key “%.*ls” with the SET NULL referential action, because one or more referencing columns are not nullable.

Severity level:
16.

Description:
This error message appears when you try to create a foreign key constraint with the referential SET NULL action, but the referencing column(s) is defined as non-nullable.

Consequences:
The T-SQL statement can be parsed, but causes the error at runtime.

Resolution:
Errors of the Severity Level 15 are generated by the user and can be fixed by the SQL Server user. The statement cannot be executed this way. Either the referential action must be changed or the referencing column must be changed to allow NULL values.

Versions:
All versions of SQL Server.

Example(s):
IF OBJECT_ID (‘dbo.T1′) IS NOT NULL
    DROP TABLE dbo.T1;
GO
IF OBJECT_ID (‘dbo.T2′) IS NOT NULL
    DROP TABLE dbo.T2;
GO

CREATE TABLE dbo.T1 (c1 int NOT NULL PRIMARY KEY);
CREATE TABLE dbo.T2 (c1 int NOT NULL PRIMARY KEY
    CONSTRAINT FK_T2_T1 FOREIGN KEY REFERENCES dbo.T1 (c1)
        ON DELETE SET NULL
);

Remarks:
In the abve example we try to create a foreign key constraint with the referential SET NULL action on dbo.T2 (c1) that references dbo.T1 (c1). Because c1 is defined as NOT NULL, the error is raised.



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