How Does Upsert Work When No External Id Is Specified?

Question

I have looked at the documentation on Database Methods. What I do not understand is, what happens when an upsert is performed without specifying an external id field? Is it just based on whether or not the record has an Id?

Answer 1

It just simply check if ID field is set then SFDC update the record orther wise it just insert record.

for ex:

List<Account> al = new list<Account>();

Account a1 = new account(ID='0019000001dK6Rd', Name='XXXX Test Account 1'); 
al.add(a1); // This is updated as ID is passed 

Account a2 = new account(MyExtID__c='Target#001', Name='XXXX Test Account 2'); 
al.add(a2);// This is updated as External ID is passed

Account a3 = new account( Name='XXXX Test Account 3'); 
al.add(a3);// This is inserted as both ID & External ID is NOT passed

Database.upsert(al); 

Update:

Usage:
Upsert is a merging of the words insert and update. This call is available for objects if the object has an external ID field or a field with the idLookup field property.

https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_calls_upsert.htm