If you have a foreign key constraint in SQL Server that is currently disabled, you can use the code below to re-enable it.
When you enable a foreign key constraint, you have the option to specify whether or not to check any existing data in the table. This also applies when you enable a CHECK constraint.
Below are code examples of enabling a foreign key constraint, while specifying each of these different options.
You can run the DBCC CHECKCONSTRAINTS console command to return a list of all constraint violations in a SQL Server database.
This command checks the integrity of a specified constraint or all constraints on a specified table in the current database. It returns any foreign key and CHECK constraint violations that it finds.
You can use the ALL_CONSTRAINTS option to check both enabled and disabled constraints. If you omit this, then only enabled constraints are returned (unless you explicitly specify a constraint to check, in which case it will be returned regardless of whether it’s enabled or disabled).
To return a list of all untrusted foreign key constraints in a SQL Server database, you can run the T-SQL code below.
An untrusted foreign key is one that has its is_not_trusted flag set to 1.
If you need to return a list of all foreign key constraints that have been disabled in a SQL Server database, you can run the T-SQL code below.
If you find yourself in the situation where you need to disable a foreign key constraint in SQL Server, here’s how to do that using Transact-SQL.
This will allow you to enter data without being restricted by the foreign key. Obviously, you wouldn’t do this unless you had a very good reason to do so. Foreign keys enforce referential integrity, so disabling them has the potential to create all sorts of issues.
If you need to return a list of all untrusted CHECK constraints in a SQL Server database, you can run the T-SQL code below.
By “untrusted”, I’m referring to those constraints that have their is_not_trusted flag set to 1.
If you need to return a list of all CHECK constraints that have been disabled in a SQL Server database, you can run the T-SQL code below.
You can use the code below to disable all CHECK and foreign key constraints for the current database in SQL Server.
EXEC sp_MSforeachtable @command1="ALTER TABLE ? NOCHECK CONSTRAINT ALL"
This uses Microsoft’s undocumented sp_MSforeachtable stored procedure. This procedure allows you to perform tasks against each table in a database. So it’s perfect for our task here – to disable all CHECK constraints within the current database.
Below is an example where I do this and then check the result.
You can use the code below to enable all CHECK and foreign key constraints for the current database in SQL Server.
When you enable a CHECK or foreign key constraint, you have the option of checking existing data in the table before the constraint is enabled. Doing this allows you to verify whether or not any existing violates the constraint. To perform this check, use WITH CHECK within the code, otherwise use WITH NOCHECK.
You can use the code below to disable all CHECK and foreign key constraints for a specific table in SQL Server.
Just replace TableName with the name of the applicable table.
ALTER TABLE TableName NOCHECK CONSTRAINT ALL
Below is an example where I do this and then check the result.