The trigger adds a default opportunity for every account that doesn’t already have an opportunity ( Bulk Query Optimized )

AddRelatedRecord_Optimized.tgr

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;
    }
}