SQL Server adds a verification constraint [failure] to a table that already has data, use the WITH NOCHECK clause

To add a constraint to an existing data table, but not to use the constraint to existing data, use the WITH NOCHECK clause when executing the ALTER TABLE statement, so that the new constraint will not constrain the existing data, but only the new data will be constrained to check:

ALTER TABLE [TableName] with Nochek add constraint [ck_xxx] Check (verification conditions)

Second, Nocheck can also disable the constraint, disable it without deleting the constraint:

ALTER TABLE [tablename] nocheck constraint [ck_xxx]

If you want to re-enable the constraint:

ALTER TABLE [tablename] CHECK constraint [ck_xxx]

SQL Server adds a verification constraint [failure] to a table that already has data, use the WITH NOCHECK clause