Overview and uses of PL/SQL triggers

Overview and uses of PL/SQL triggers

In a database, a trigger is a special stored procedure that automatically executes when a specific database event occurs. Triggers can be used to execute custom logic before or after data is inserted, updated, or deleted. They provide a way to implement business rules and data integrity at the database level.

Triggers are usually used in the following situations:

• Data integrity constraints: Triggers can be used to check the validity of data before inserting, updating, or deleting it, and prevent operations that do not comply with business rules.
• Data change recording: Triggers can be used to record change history or generate relevant audit information when data changes.
• Data derived calculations: Triggers can be used to automatically calculate and update related derived data when data is inserted, updated, or deleted.
• Data replication and synchronization: Triggers can be used to automatically replicate changes to other databases or synchronize data when data changes.

Creation and triggering timing of PL/SQL triggers

In PL/SQL, you can use the CREATE TRIGGER statement to create triggers. To create a trigger, you need to specify the name of the trigger, the associated table name, and the triggering time. The trigger timing can be BEFORE (the trigger is executed before the trigger event) or AFTER (the trigger is executed after the trigger event).

Here is an example of creating a trigger:

-- 创建触发器
CREATE OR REPLACE TRIGGER trigger_name
BEFORE INSERT OR UPDATE OR DELETE ON table_name
FOR EACH ROW
BEGIN
  -- 触发器的操作
  -- ...
END;
/

In the above example, we created a trigger named trigger_name using the CREATE TRIGGER statement. The trigger will execute before or after an INSERT, UPDATE, or DELETE operation on the table_name table. FOR EACH ROW means that the trigger will execute once for each row of data.

Trigger events and trigger conditions of PL/SQL triggers

The firing event of a trigger refers to the specific operation performed by the trigger, usually associated with the INSERT, UPDATE, or DELETE operation on the table. Trigger conditions refer to the conditions for trigger execution, which determine whether the trigger is executed. The trigger condition can be a Boolean expression, and the trigger executes when the expression is true, otherwise the trigger does not execute.

Here is an example that demonstrates a trigger that validates data before inserting a new record:

-- 创建触发器
CREATE OR REPLACE TRIGGER validate_data
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
  -- 检查数据的有效性
  IF :NEW.salary < 0 THEN
    RAISE_APPLICATION_ERROR(-20001, 'Salary cannot be negative');
  END IF;
END;
/

In the above example, we created a trigger called validate_data. This trigger executes before the INSERT operation on the employees table. In the action part of the trigger, we use an IF statement to check if the salary of the newly inserted record is less than 0. If the salary is less than 0, use the RAISE_APPLICATION_ERROR function to throw a custom application error.

Exception handling and writing specifications for PL/SQL triggers

In the action part of the trigger, we can use exception handling to handle errors that may occur. Exceptions can be caught and handled using the EXCEPTION keyword and associated exception handling blocks.

Here is an example that demonstrates how to handle exceptions in a trigger:

-- 创建触发器
CREATE OR REPLACE TRIGGER handle_exception
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
  -- 检查数据的有效性
  IF :NEW.salary < 0 THEN
    RAISE_APPLICATION_ERROR(-20001, 'Salary cannot be negative');
  END IF;
EXCEPTION
  WHEN OTHERS THEN
    -- 处理异常
    DBMS_OUTPUT.PUT_LINE('An error occurred: ' || SQLERRM);
END;
/

In the above example, if the conditions in the trigger are not met, a custom application error will be thrown. In the EXCEPTION block, we use the DBMS_OUTPUT.PUT_LINE function to print the error message to the console.

When writing triggers, there are some coding conventions you can follow to ensure the readability and maintainability of your code:

1. Use meaningful trigger names: Give your trigger a descriptive name so that its function and purpose are clearly understood.
2. Add comments: Add comments in the trigger code to explain the purpose, logic and usage of the trigger.
3. Use consistent indentation and formatting: Use a consistent indentation and formatting style to improve code readability.
4. Try to avoid overly complex trigger logic: Complex trigger logic may be difficult to understand and maintain, so try to keep triggers simple and easy to understand.

By using PL/SQL triggers, we can implement business rules and data integrity at the database level while improving data consistency and reliability. Triggers can automatically execute customized logic, thereby reducing the workload of manual data processing and improving database performance and security.