Shulou(Shulou.com)06/01 Report--

To better handle situations where data may temporarily violate constraints, you can specify constraints as different states. You can enable (ENABLE) or disable (DISABLE) integrity constraints. If constraints are enabled, the data is checked when it is entered or updated in the database. At this point, it is forbidden to enter data that does not meet the constraint rules. If you disable constraints, you can enter data that does not conform to the rules in the database. Integrity constraints can be in one of the following states:

1. DISABLE NOVALIDATE: new and existing data are not checked, so these data may not meet the constraints. This state is usually used when the data comes from a validated source and the table is read-only. Therefore, new data is not entered into the table.

2. DISABLE VALIDATE: if the constraint is in this state, no modifications to constrained columns are allowed. Because if, after validating existing data, unchecked data is allowed to be entered into the table, inconsistencies will occur. This state is usually used when existing data must be validated, but the data does not need to be modified, and no additional indexing is required for performance.

3. ENABLE NOVALIDATE: the new data meets the constraints, but the existing data is in an unknown state. This state is often used when existing constraint violations can be corrected while new violation data is not allowed to be entered into the system.

4. ENABLE VALIDATE: both the new data and the existing data meet the constraint conditions. This is the typical and default state of a constraint.

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