Pretriggers

Pretriggers, also called before triggers, are a type of trigger that fires in response to a data modification attempt or event before the action is completed. They are supported by many database management systems and by some event-driven frameworks. The primary purpose is to enforce rules, transform input data, or make decisions before the system commits changes.

In a typical relational DBMS, a pretrigger can be defined for operations such as INSERT, UPDATE, or DELETE and can be row-level or statement-level. It has access to the proposed new row values (often named NEW) and the existing row values (OLD). In a row-level pretrigger, modifications can be applied to NEW, and returning a modified NEW or aborting by raising an error may alter or cancel the operation; in many systems, returning NULL or raising an exception cancels the action.

Common uses include data validation that cannot be expressed with constraints, normalization of values, automatic population of derived fields, enforcing business rules, and performing authorization checks before changes proceed. Pretriggers differ from post-triggers, which run after the data change and are often used for auditing or logging.

Implementation notes: the behavior of pretriggers varies by system; they execute within the same transaction and may impact performance. Recursion risks and mutating-table restrictions require careful design. In some environments, pretriggers are complemented by other mechanisms such as constraints, access controls, or application-level logic.