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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.
The FOREIGN KEY constraint is a key used to link two tables together. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
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;
This tutorial helps you understand SQL foreign key and show you how to define a foreign key using the FOREIGN KEY constraints.
22 lip 2024 · To successfully change or delete a row in a foreign key constraint, you must first either delete the foreign key data in the foreign key table or change the foreign key data in the foreign key table, which links the foreign key to different primary key data.
To create a new table containing a foreign key column that references another table, use the keyword FOREIGN KEY REFERENCES at the end of the definition of that column. Follow that with the name of the referenced table and the name of the referenced column in parentheses.
26 wrz 2024 · When a foreign key contains a NULL, SQL Server does not require the corresponding value in the parent table, allowing the row to exist without violating the foreign key constraint. Example 1: Foreign Key with NULL Values.
