How to set SQL trigger

A SQL trigger is a database object that automatically performs specific actions when a specific event is executed on a specified table. To set up SQL triggers, you can use the CREATE TRIGGER statement, which includes the trigger name, table name, event type, and trigger code. The trigger code is defined using the AS keyword and contains SQL or PL/SQL statements or blocks. By specifying trigger conditions, you can use the WHERE clause to limit the execution scope of a trigger. Trigger operations can be performed in the trigger code using the INSERT INTO, UPDATE, or DELETE statement. NEW and OLD keywords can be used to reference the affected keyword in trigger code.

How to set SQL triggers

1. What are SQL triggers?

SQL triggers are database objects that automatically perform specific actions when specific events are performed on specified tables. It is used to maintain data integrity and consistency.

2. How to set SQL triggers

The syntax for creating a trigger is as follows:

 <code class="sql">CREATE TRIGGER `触发器名称` ON `表名称` FOR `事件类型` AS `触发器代码`;</code>

in:

• 触发器名称: The unique name of the trigger.
• 表名称: The table where the trigger is applied.
• 事件类型: Events that trigger the trigger, such as INSERT, UPDATE, or DELETE.
• 触发器代码: SQL statements or PL/SQL code blocks executed after the trigger event occurs.

3. Detailed steps

1. Create triggers using the CREATE TRIGGER statement.
2. Specifies which table to create the trigger and which event to trigger.
3. Use the AS keyword followed by SQL or PL/SQL code blocks.
4. Encapsulate trigger code using BEGIN and END statements.
5. Specifies a trigger condition, such as the WHERE clause, to limit the execution of certain rows of the trigger.
6. Use INSERT INTO , UPDATE , or DELETE statement to perform the trigger operation.
7. Use NEW and OLD keywords in the trigger code to reference the value of the affected row.
8. Use COMMIT statement to submit the transaction to activate the trigger.

Example:

 <code class="sql">-- 创建一个在表"orders" 上插入记录时执行的触发器CREATE TRIGGER `order_trigger` ON `orders` FOR INSERT AS -- 插入新行到"order_log" 表INSERT INTO `order_log` (`order_id`, `order_date`, `action`) VALUES (NEW.`order_id`, NEW.`order_date`, 'INSERTED');</code>

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