SQL trigger functions and syntax

The use and syntax of SQL triggers

Introduction:
SQL trigger (Trigger) is a special stored procedure that will occur in a database table Automatically triggered when specific operations occur. Triggers enable automated processing of table data update, insertion, and deletion operations. This article will introduce the use and syntax of SQL triggers, and provide specific code examples.

1. The concept and function of SQL triggers
SQL triggers are a very important part of the database. They can be automatically triggered when specific data operations (such as INSERT, UPDATE, DELETE) occur. . Through triggers, operations such as constraining, checking, supplementing or modifying table data can be implemented. The functions of SQL triggers include but are not limited to the following:

1. Data constraints: Data constraints can be achieved through triggers to ensure data integrity and consistency.
2. Automated processing: You can automatically perform specified operations when a trigger operation occurs, such as generating logs, updating related data, etc.
3. Data supplement: You can insert, update, and delete relevant data in specified operations through triggers to meet the needs of data supplement or modification.

2. SQL trigger syntax
SQL trigger syntax can be defined through the CREATE TRIGGER statement. Its basic syntax structure is as follows:

CREATE TRIGGER trigger_name
{BEFORE | AFTER} {INSERT | UPDATE | DELETE}
ON table_name
FOR EACH ROW
trigger_body

where:

• trigger_name is the name of the trigger , can be customized according to needs.
• The BEFORE and AFTER keywords are used to specify that the trigger triggers execution before or after the operation.
• INSERT, UPDATE and DELETE keywords are used to specify what kind of operation the trigger processes.
• table_name is the name of the table targeted by the trigger.
• FOR EACH ROW keyword indicates that the operation of each row of data will trigger the execution trigger.
• trigger_body is the specific execution logic of the trigger, which can be a PL/SQL code block.

3. Specific code examples
The following uses a specific example to illustrate the use of SQL triggers:

Suppose there is a table named "Customers", which contains The following fields:

CustomerID (primary key)
CustomerName
Address
City
Country

We need to automatically update each time an INSERT operation is performed on the table The "LastUpdated" field is the current time. This can be achieved with the following trigger:

CREATE TRIGGER update_last_updated
AFTER INSERT ON Customers
FOR EACH ROW
BEGIN
UPDATE Customers
SET LastUpdated = CURRENT_TIMESTAMP
WHERE CustomerID = :NEW.CustomerID;
END;

In the above code, "update_last_updated" is the name of the trigger, AFTER INSERT means triggering execution after the insert operation, Customers is the name of the table, FOR EACH ROW means triggering execution for each row of data. The trigger_body part is a PL/SQL code block, in which the value of the "LastUpdated" field is updated to the current time through the UPDATE statement.

Through the above trigger, every time an INSERT operation is performed on the "Customers" table, the "LastUpdated" field will be automatically updated to the current time.

Conclusion:
SQL triggers provide a convenient way to automate the processing of database table data. By defining triggers, you can automatically execute specified logic code when a specific operation occurs. This article introduces the usage and syntax of SQL triggers, and provides specific code examples. I hope it will be helpful to readers in understanding and applying SQL triggers.

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