SQL Server isn’t validating composite foreign key constraints when NULLs or mismatched values appear. For example:
DROP TABLE IF EXISTS UserRecords;
DROP TABLE IF EXISTS Regions;
DROP TABLE IF EXISTS Countries;
CREATE TABLE Countries (id INT PRIMARY KEY);
CREATE TABLE Regions (id INT, code NVARCHAR(10), PRIMARY KEY(id, code));
CREATE TABLE UserRecords (uid INT IDENTITY PRIMARY KEY, id INT, code NVARCHAR(10));
ALTER TABLE Regions ADD CONSTRAINT FK_Regions_Countries FOREIGN KEY(id) REFERENCES Countries(id);
ALTER TABLE UserRecords ADD CONSTRAINT FK_UserRecords_Countries FOREIGN KEY(id) REFERENCES Countries(id);
ALTER TABLE UserRecords ADD CONSTRAINT FK_UserRecords_Regions FOREIGN KEY(id, code) REFERENCES Regions(id, code);
In my experience, composite foreign keys in SQL Server work properly when all columns are consistently defined and not left to unpredictable behaviors with NULL values. Problems often occur when one of the columns is nullable, causing implicit bypassing of the constraint. I resolved similar issues by reviewing column definitions and enforcing non-nullability where necessary. Ensuring that both sides of the foreign key relationship have matching and non-null columns improves data integrity checks and prevents unexpected errors during operations like data insertion.
i ran into this issue before. the fix was to make sure the columns in the fk were not nullable and had matching types. sometimes even a minor mismatch kills the integrity check. double-checking schema definitions helped resolve my problems.
hey, this is intriguing. did someone try using extra chek constraints on composite fks? i’m curious if sql server ver or settings play a part. what do others think about alternative schema tweaks to avoid these intermittent misses?