SQLite Triggers - After Update

Triggers are special types of procedures in a database management system that execute automatically in response to certain events such as an update, insert or delete operation on a table. SQLite supports triggers, and one type of trigger is an "after update" trigger. This trigger is activated after an update operation on the table.

Syntax

The SQL syntax for an after update trigger in SQLite is as follows:

CREATE TRIGGER trigger_name 
AFTER UPDATE ON table_name
FOR EACH ROW
BEGIN
    -- code to execute
END;

Example

Suppose we have a table called users with columns for id, name, and age. We want to create an after update trigger that logs the name and age of all users whose records have been updated.

CREATE TRIGGER log_update
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
    INSERT INTO log_table (user_name, user_age) 
    VALUES (OLD.name, OLD.age);
END;

Output

The output of running the log_update trigger would be to insert a row into the log_table for every update to the users table, with the user_name and user_age fields populated with the name and age values of the updated row.

Explanation

In the example above, we create a trigger called log_update that is triggered after an update operation is performed on the users table. For each row that is updated in the table, the trigger inserts a row into the log_table with the user name and user age fields set to the name and age values of the updated row, respectively.

Use

The after update trigger can be used to perform tasks such as logging changes to a table, maintaining audit trails, or updating data in other tables based on changes to a table.

Important Points

• After update triggers are executed after an update operation on the table.
• The OLD and NEW keywords are used within the trigger to refer to the old and new values of the row being updated.
• Triggers can execute complex SQL statements, including queries and subqueries.
• Be careful when using triggers, as they can impact performance if not implemented properly.

Summary

In this tutorial, we learned about after update triggers in SQLite and saw an example of how to create a trigger that logs data from a table after an update operation is performed. After update triggers can be used to perform a wide range of tasks such as maintaining audit trails or updating data in other tables based on changes to a table. Keep in mind that triggers can impact performance if not implemented properly.