The use and syntax of sql triggers

A trigger is a database code that executes automatically when a specific event (insert, update, delete) occurs. The trigger syntax includes trigger name, table name, triggering time (BEFORE/AFTER) and event type (INSERT/UPDATE/DELETE). Trigger types include BEFORE and AFTER, and event types include INSERT, UPDATE, and DELETE. Triggers can be used for data integrity verification, audit logging, automated tasks, and business logic. For example, you can create a trigger to record the insertion time when a new row is inserted to ensure data consistency.

SQL trigger usage and syntax

What is a trigger?

A trigger is a piece of code in the database that automatically triggers execution when a specific event occurs in the database. They are used to perform custom actions when data changes.

The syntax of triggers

The syntax of triggers in SQL is as follows:

<code>CREATE TRIGGER trigger_name
ON table_name
FOR INSERT | UPDATE | DELETE
AS
BEGIN
   -- 触发器代码...
END;</code>

Types of triggers

According to the triggering timing, triggers can be divided into the following types:

• BEFORE Trigger: executed before the event occurs.
• AFTER Trigger: executed after the event occurs.

According to the event type, triggers can be divided into:

• INSERT Trigger: triggered when a new row is inserted.
• UPDATE Trigger: Fires when an existing row is updated.
• DELETE Trigger: Fires when a row is deleted.

Usage scenarios of triggers

Triggers are widely used in the following scenarios:

• Data integrity verification : Ensure that the data conforms to specific rules (for example, non-null constraints, unique constraints).
• Auditing and Logging: Track changes in the database and log user activity.
• Automated tasks: Automatically perform specific tasks (e.g., send email notifications) when data changes.
• Business logic: Implement complex business rules, such as calculating derived columns or maintaining related tables.

Example

The following is an example trigger that logs the insertion time when a new row is inserted into the users table:

<code class="sql">CREATE TRIGGER insert_timestamp
BEFORE INSERT ON users
AS
BEGIN
   SET NEW.created_at = CURRENT_TIMESTAMP();
END;</code>

When inserting a new row into the users table, the insert_timestamp trigger will be executed before the insertion and automatically set the current timestamp to the ## of the new row. #created_at field.

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