To add a constraint to an existing data table using an SQL statement

If you add a constraint to the table where the data exists, it is possible that the data does not conform to the CHECK constraint and the add constraint fails.

Such as:

This is a table that adds a check constraint for the ID number.

Use demo--points to the database of the current operation Goalter  TABLE employeeadd  constralnt ck_employeeid CHECK (LEN (employeeid=18)

Then it will appear

The error means that the data table contains the ID number of the existing data is not equal to 18, and check constraints conflict, will add failure.

Use alter TABLE employee+ with Nocheck to resolve.

With Nocheck says:

Do not check the existing data in the table, only check the data entered after adding the constraint.

To add a constraint to an existing data table using an SQL statement