Trigger Syntax

The syntax of a trigger definition is different from a class definition’s syntax. A trigger definition starts with the trigger keyword. It is then followed by the name of the trigger, the Salesforce object that the trigger is associated with, and the conditions under which it fires. A trigger has the following syntax:

trigger TriggerName on ObjectName (trigger_events) { code_block }

To execute a trigger before or after insert, update, delete, and undelete operations, specify multiple trigger events in a comma-separated list. The events you can specify are:

• before insert
• before update
• before delete
• after insert
• after update
• after delete
• after undelete

Types of Triggers

There are two types of triggers.

• Before triggers are used to update or validate record values before they’re saved to the database.
• After triggers are used to access field values that are set by the system (such as a record's Id  or LastModifiedDate  field), and to affect changes in other records. The records that fire the after trigger are read-only.

Using Context Variables

To access the records that caused the trigger to fire, use context variables. For example, Trigger.New contains all the records that were inserted in insert or update triggers. Trigger.Old provides the old version of sObjects before they were updated in update triggers, or a list of deleted sObjects in delete triggers. Triggers can fire when one record is inserted, or when many records are inserted in bulk via the API or Apex. Therefore, context variables, such as Trigger.New, can contain only one record or multiple records. You can iterate over Trigger.New to get each individual sObject.

This example is a modified version of the HelloWorldTrigger example trigger. It iterates over each account in a for loop and updates the Description field for each.

trigger HelloWorldTrigger on Account (before insert) { 
    for(Account a : Trigger.New) { 
		    a.Description = 'New description'; 
	  }
}