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Foreign keys mean the value must exist in the other table. NULL refers to the absence of value, so when you set a column to NULL, it wouldn't make sense to try to enforce constraints on that.
In your case, a value would be a foreign key that references a primary key elsewhere. A null would indicate the absence of a value, just as it always does. At the next level of abstraction, foreign keys represent relationships.
26 wrz 2024 · Yes, a foreign key can be NULL in SQL Server. Unlike primary keys, which cannot contain NULL values because they must uniquely identify each row, foreign keys have no such restriction. NULL values in foreign keys represent “unknown” or “unassociated” records.
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;
12 maj 2020 · SQL Server sees NULL as "unknown", rather than as a specific value, so assigning meaning to NULL is going against the normal behaviour of the engine. It is the reason you cannot compare two NULL values with = in SQL Server.
22 lip 2024 · This article describes how to create foreign key relationships in SQL Server by using SQL Server Management Studio or Transact-SQL. You create a relationship between two tables when you want to associate rows of one table with rows of another.
10 paź 2023 · You can not use null in a foreign key because null means no match (empty set). Just define an unspecified value.
