Creates opportunity record if the account doesn't contain at least one opportunity.

trigger oneopp on Account (after update) {
    Opportunity[] newOpportunities = new Opportunity[0];
    Account[] potentialAccounts = new Account[0];
    for(Account record: Trigger.new) {
        if(Trigger.oldMap.get(record.Id).AnnualRevenue != record.AnnualRevenue) {
            potentialAccounts.add(record);
        }
    }
    if(potentialAccounts.isEmpty()) {
        return;
    }
    Set<Id> accountIdsWithOpps = new Map<Id, AggregateResult>([
        SELECT COUNT(Id), AccountId Id 
        FROM Opportunity
        WHERE AccountId = :potentialAccounts
        GROUP BY AccountId
    ]).keySet();
    for(Account record: potentialAccounts) {
        if(!accountIdsWithOpps.contains(record.Id)) {
            newOpportunities.add(
                new Opportunity(Name='dummyopp '+record.Name, AccountId = record.Id, CloseDate=Date.Today(), StageName='Closed Lost', Amount=record.AnnualRevenue)
            );
        }
    }
    insert newOpportunities;
}