Triggers are a fundamental component of Oracle PL/SQL, and they allow developers to define actions that automatically execute in response to certain events on a particular table or view. Before triggers are a type of trigger that fires before the triggering event, providing developers with the opportunity to intervene and modify data before the action takes place. Let’s delve into the key aspects of Oracle PL/SQL before triggers.
Structure of a Before Trigger:
A before trigger is associated with a specific event, such as an INSERT, UPDATE, or DELETE statement, and it executes before the actual event takes place. The basic structure of a before trigger in PL/SQL includes the following components:
CREATE OR REPLACE TRIGGER trigger_name BEFORE INSERT OR UPDATE OR DELETE ON table_name FOR EACH ROW DECLARE -- Declaration of local variables and constants BEGIN -- PL/SQL code to be executed before the triggering event -- Accessing and modifying data using :OLD and :NEW pseudorecords :NEW.column_name := value; -- Additional PL/SQL logic END; /
trigger_name: The unique identifier for the trigger.
BEFORE INSERT OR UPDATE OR DELETE: Specifies the triggering event.
ON table_name: Specifies the table on which the trigger operates.
FOR EACH ROW: Indicates that the trigger is row-level, meaning it executes once for each affected row.
Common Use Cases
Data Validation: Before triggers are often used to validate data before it is inserted or updated in the database. Developers can enforce business rules, data integrity, and consistency checks within the trigger.
Default Values: You can use before triggers to set default values for certain columns if they are not explicitly provided in an INSERT statement.
Audit Trails: Before triggers are valuable for maintaining audit trails. Changes to data can be logged or recorded in another table for tracking purposes.
Pseudorecords: :OLD and :NEW
Inside a before trigger, developers have access to two special pseudorecords: :OLD and :NEW. These records represent the old and new values of the row being affected by the triggering event.
:OLD.column_name: Represents the current value of the specified column before the triggering event.
:NEW.column_name: Represents the new value that the column will have after the triggering event.
CREATE OR REPLACE TRIGGER before_insert_example BEFORE INSERT ON employees FOR EACH ROW BEGIN -- Set a default value for the hire_date if not provided IF :NEW.hire_date IS NULL THEN :NEW.hire_date := SYSDATE; END IF; END; /
In this example, the before trigger ensures that the hire_date column is set to the current date and time if it is not provided in the INSERT statement.
Before triggers in Oracle PL/SQL offer a robust mechanism for customizing and controlling the data modification process, allowing developers to enforce business rules, maintain data integrity, and implement complex logic before database changes occur.