When a check constraint is added to an existing table, the check constraint can apply to either,
To new data only
By default the check constraints applies to new data as well as existing data,
ALTER TABLE ADD CONSTRAINTS CHECK(condition)
Adding to new data only:
ALTER TABLE with no check ADD CONSTRAINTS CHECK(condition)
You will be careful when adding a constraint without checking the existing data, because this bypasses the controls
in Microsoft SQL Server 2000 that enforces the integrity rules for the table.