A trigger in SQLite is a named database object that is executed automatically when an INSERT, UPDATE or DELETE statement is issued against the associated table. Triggers are useful for tasks such as enforcing business rules, validating input data, and keeping an audit trail.

To create a trigger in SQLite, you use the CREATE TRIGGER statement. The syntax for the CREATE TRIGGER statement is as follows:

Code snippet

CREATE TRIGGER trigger_name
BEFORE | AFTER
INSERT | UPDATE | DELETE
ON table_name
[FOR EACH ROW]
BEGIN
  -- trigger body
END;

The trigger_name is the name of the trigger. The BEFORE or AFTER keyword specifies when the trigger will be executed. The INSERT, UPDATE, or DELETE keyword specifies the type of event that will trigger the trigger. The table_name is the name of the table that the trigger is associated with. The FOR EACH ROW clause specifies that the trigger will be executed once for each row that is affected by the event.

The trigger body is the code that will be executed when the trigger is triggered. The trigger body can contain any valid SQL statements.

For example, the following trigger will prevent any user from inserting a value into the salary column that is greater than $100,000:

Code snippet

CREATE TRIGGER salary_check
BEFORE INSERT ON employees
FOR EACH ROW
BEGIN
  IF NEW.salary > 100000 THEN
    RAISE EXCEPTION 'Salary cannot be greater than $100,000';
  END IF;
END;

When a user tries to insert a value into the salary column that is greater than $100,000, the trigger will be triggered and the exception will be raised. The user will then be prompted to enter a different value for the salary.

Triggers can be used to perform a variety of tasks, such as:

• Enforcing business rules
• Validating input data
• Keeping an audit trail
• Replicating data to different tables
• Executing complex queries

Triggers can be a powerful tool for managing your data. By using triggers, you can automate tasks and ensure that your data is always consistent.