Which clause is used while creation of foreign key?
A foreign key is a column or group of columns in one table that contains values that match the primary key values in the same or another table. Each foreign key is defined using the FOREIGN KEY clause combined with the REFERENCES clause.
Can Unique Key be a foreign key?
Yes, why not. It is possible to reference a UNIQUE constraint in a FOREIGN KEY . You could have a Primary key and an Unique key, and you would like to validate both. Yes, you can reference a column (or columns) governed by either a primary key constraint or a unique constraint.
Can a foreign key be created with nocheck?
Foreign key created using NOCHECK option was created as an untrusted one which means that it won’t be used by query optimizer when building the most effective execution plan later. We can verify that fact in metadata:
What do you need to know about with nocheck?
By the way, these arguments also apply to foreign key constraints. As you might expect, WITH CHECK specifies that existing data is validated and WITH NOCHECK specifies that it isn’t. The default is WITH NOCHECK. If you use WITH NOCHECK, the constraint will be flagged as untrusted.
What should you know about with nocheck when enabling a constraint?
In other words: When you use WITH NOCHECK, the constraint will remain untrusted. When you use WITH CHECK it will become trusted, but only if all existing data conforms to the constraint. If any existing data violates the constraint, then the constraint will not be enabled and you’ll receive an error message.
What happens if nocheck is not set in SQL Server?
If NOCHECK is not set then SQL Server presumes that anything that is in that column definitely exists because the entry could not exist in the table if it was not already a primary key, and you couldn’t delete the primary key without deleting the row in question. Simply NOCHECK is a foreign key that you can’t trust to actually relate to anything.