20.1.5.1 Adding an Error Message in PL/SQL

Use APEX_ERROR.ADD_ERROR to add an error message from server-side processing.

It halts further processing, abandons data changes, and shows the error in the notification area.

• Enforcing Rules with Read-Consistency
  Use a page process after the data-saving processes to validate pending changes before APEX commits them.
• Applying Aggregate Constraints with SQL
  Use SQL aggregation in a page process after the data-saving processes to reject changes that violate cross-row rules.