Apex Bulk Trigger Design Patterns

trigger SoqlTriggerBulk on Account(after update) {
    // Perform SOQL query once.  
    // Get the accounts and their related opportunities.
    List<Account> acctsWithOpps =
        [SELECT Id,(SELECT Id,Name,CloseDate FROM Opportunities)
         FROM Account WHERE Id IN :Trigger.New];

    // Iterate over the returned accounts  
    for(Account a : acctsWithOpps) {
        Opportunity[] relatedOpps = a.Opportunities;
        // Do some other processing
    }
}

trigger SoqlTriggerBulk on Account(after update) {
    // Perform SOQL query once.  
    // Get the related opportunities for the accounts in this trigger.
    List<Opportunity> relatedOpps = [SELECT Id,Name,CloseDate FROM Opportunity
        WHERE AccountId IN :Trigger.New];

    // Iterate over the related opportunities  
    for(Opportunity opp : relatedOpps) {
        // Do some other processing
    }
}

trigger SoqlTriggerBulk on Account(after update) {  
    // Perform SOQL query once.    
    // Get the related opportunities for the accounts in this trigger,
    // and iterate over those records.
    for(Opportunity opp : [SELECT Id,Name,CloseDate FROM Opportunity
        WHERE AccountId IN :Trigger.New]) {
  
        // Do some other processing
    }
}

Triggers execute on batches of 200 records at a time. So if 400 records cause a trigger to fire, the trigger fires twice, once for each 200 records. For this reason, you don’t get the benefit of SOQL for loop record batching in triggers, because triggers batch up records as well. The SOQL for loop is called twice in this example, but a standalone SOQL query would also be called twice. However, the SOQL for loop still looks more elegant than iterating over a collection variable!

trigger DmlTriggerNotBulk on Account(after update) {   
    // Get the related opportunities for the accounts in this trigger.        
    List<Opportunity> relatedOpps = [SELECT Id,Name,Probability FROM Opportunity
        WHERE AccountId IN :Trigger.New];          

    // Iterate over the related opportunities
    for(Opportunity opp : relatedOpps) {      
        // Update the description when probability is greater 
        // than 50% but less than 100% 
        if ((opp.Probability >= 50) && (opp.Probability < 100)) {
            opp.Description = 'New description for opportunity.';
            // Update once for each opportunity -- not efficient!
            update opp;
        }
    }    

}

trigger DmlTriggerBulk on Account(after update) {   
    // Get the related opportunities for the accounts in this trigger.        
    List<Opportunity> relatedOpps = [SELECT Id,Name,Probability FROM Opportunity
        WHERE AccountId IN :Trigger.New];
          
    List<Opportunity> oppsToUpdate = new List<Opportunity>();

    // Iterate over the related opportunities
    for(Opportunity opp : relatedOpps) {      
        // Update the description when probability is greater 
        // than 50% but less than 100% 
        if ((opp.Probability >= 50) && (opp.Probability < 100)) {
            opp.Description = 'New description for opportunity.';
            oppsToUpdate.add(opp);
        }
    }
    
    // Perform DML on a collection
    update oppsToUpdate;

}

trigger AddRelatedRecord on Account (after insert, after update) {
List<Opportunity> oppList = new List<Opportunity>();
    
    // Add an opportunity for each account if it doesn't already have one.
    // Iterate over accounts that are in this trigger but that don't have opportunities.
    for (Account a : [SELECT Id,Name FROM Account
                     WHERE Id IN :Trigger.New AND
                     Id NOT IN (SELECT AccountId FROM Opportunity)]) {
        // Add a default opportunity for this account
        oppList.add(new Opportunity(Name=a.Name + ' Opportunity',
                                   StageName='Prospecting',
                                   CloseDate=System.today().addMonths(1),
                                   AccountId=a.Id)); 
    }
    
    if (oppList.size() > 0) {
        insert oppList;
    }

}

trigger ClosedOpportunityTrigger on Opportunity (after insert, after update) {
List<Task> taskList = new List<Task>();
    
    // Assign a Task to an opportunity, whose StageName == 'Closed Won'.
    for (Opportunity o : Trigger.new) {
        if(o.StageName == 'Closed Won'){
            // Add a task to the opportunity
        taskList.add(new Task(Subject='Follow Up Test Task', WhatId=o.Id)); 
        }
    }
    
    if (taskList.size() > 0) {
        insert taskList;
    }

}