[SalesForce] How to check is Price Book added in the Opportunity or not in, Apex class, Salesforce

Conceptually, there is one Standard Pricebook and there are many custom Pricebooks. This allows you to do the following:

• For SKU xyz, the standard pricebook price could be $10.00
• For the same SKU xyz, on the pricebook called Commercial, the price is $10.00
• For the same SKU xyz, on the pricebook called US Government, the price is only $8.00

When you create a pricebookEntry for the junction between Product2 and Standard Pricebook, set useStandardPricebook to false. All pricebookEntries on standard Pricebook have useStandardPrice = false

When you create a pricebookEntry for the junction between Product2 xyz and Pricebook US Government, set useStandardPricebook to false and set unitPrice to 8.00.

On custom pricebook called 'Commercial' where SKU xyz is $10.00, then when you create a pricebookEntry for the junction between Product2 xyz and Pricebook Commercial, set useStandardPricebook to true and set unitPrice to 10.00.

In effect, your updates to PricebookEntry are mimicing the SFDC Force.com user interface where you define standard pricebook prices for a Product and then, if you use custom pricebooks, you can choose to use the standard price or override the standard price, just for that pricebook.

Don't forget that all pricebook entries must be inserted against the Standard pricebook before you can insert pricebookEntries on custom pricebooks.

As the error states, you need to specify the opportunity id. Presumably, you'll find the opportunity attached to the order.

That probably looks like this:

trigger CreateOpportunityProduct on Order_Line_Item__c(after insert, after update){       
if(trigger.isUpdate)
{          
    map<id, order__c> orders = new map<id, order__c>();
List<ID> pricebookID=new List<ID>();       
List<ID> opportunityID=new List<ID>();           
List<OpportunityLineItem> newopportunitylineitemlist=new List< OpportunityLineItem>();        
    for(order_line_item__c record: trigger.new) {
        orders.put(record.order__c, null);
    }
    orders.putall([select id, opportunity__c from order__c where id in :orders.keyset()]);
for(Order_Line_Item__c orderlist: Trigger.new){    
OpportunityLineItem Oppitem=new OpportunityLineItem();
Oppitem.Quantity=orderlist.Quantity__c;
Oppitem.UnitPrice=orderlist.UnitPrice__c;
oppitem.opportunityid = orders.get(orderlist.order__c).opportunity__c;
pricebookID.add(Oppitem.PriceBookEntryId);
opportunityID.add(Oppitem.OpportunityId);
newopportunitylineitemlist.add(Oppitem);
}    
List<Order__c> orderlist= new List<Order__c>([Select Id,Opportunities__c From Order__c Where Opportunities__c IN:opportunityID]);            
List<PricebookEntry> pricebookentrylist = new List<PricebookEntry>([Select Id,Product2Id From PricebookEntry Where Id IN:pricebookID]);         
if(newopportunitylineitemlist.size()>0)
insert(newopportunitylineitemlist);    
}    
}

Adjust field names as necessary.