How to build a SQL trigger

SQL triggers are database objects that automatically perform operations when a specific event occurs. Create a trigger consists of the following steps: Select the trigger type: BEFORE, AFTER, or INSTEAD OF Specify the trigger event: INSERT, UPDATE, or DELETE Write the trigger code, specify the operation to be performed to create the trigger using the CREATE TRIGGER statement.

How to create SQL triggers

SQL triggers are database objects that automatically trigger a set of operations when a specific event occurs. For example, perform an action when inserting a new row into a table.

Create a trigger

To create a trigger, you can use the following steps:

1. Select a trigger type : SQL supports the following types of triggers:

   • BEFORE : Triggered before the event is executed
   • AFTER : Triggered after the execution event
   • INSTEAD OF : Alternative trigger event

2. Specify a trigger event : The trigger can respond to the following events:

   • INSERT : Insert a new row into the table
   • UPDATE : Update existing rows in the table
   • DELETE : Delete rows from table
3. Write trigger code : The trigger code specifies the action to be performed. This can include inserting new rows, updating existing rows, or writing to other tables.
4. Create a trigger : Create a trigger using the CREATE TRIGGER statement. The syntax is as follows:

 <code class="sql">CREATE TRIGGER [触发器名称] ON [表名称] FOR [触发事件] AS BEGIN [触发器代码] END;</code>

Example

The following example creates a BEFORE INSERT trigger that inserts the current timestamp before inserting a new row into the "Customer" table:

 <code class="sql">CREATE TRIGGER before_customer_insert ON customers FOR INSERT AS BEGIN UPDATE customers SET created_at = CURRENT_TIMESTAMP WHERE id = NEW.id; END;</code>

Notice :

• The trigger name must be unique.
• The table name must be the table to which the trigger will act.
• Trigger event specifies the trigger condition for the trigger.
• The BEGIN and END statements mark the start and end of the trigger code block.
• NEW and OLD expressions refer to newly inserted or updated rows and old rows, respectively.
• Trigger code can contain SQL statements, process control structures, and functions.

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