7-17. Constraint States

To better deal with situations where data must be temporarily in violation of a constraint, you can designate a constraint to be in various states.
An integrity constraint can be ENABLED or DISABLED. This applies to newly entered data.
An integrity constraint can VALIDATE or NOVALIDATE existing data.
To sumarize, an integrity constraint can be in one of the four states:

• DISABLE NOVALIDATE (often used when the data is from an already validated source and the table is read-only, so no new data is being entered into the table).
• DISABLE VALIDATE (used when the existing data must be validated but the data is not going to be modified and the index is not otherwise needed for performance).
• ENABLE NOVALIDATE (used so that existing constraint violations can be corrected, and at the same time, new violations are not allowed to enter the system).
• ENABLE VALIDATE (both new and existing data must conform to the constraint)