Upsert and External IDs

Account acc = new Account(Name = 'Riverbend Farms');
upsert acc; // no Id yet — this inserts

acc.Industry = 'Agriculture';
upsert acc; // now has an Id — this updates

The exact same statement, upsert acc, does an insert the first time (no Id present) and an update the second time (an Id now exists) — no need to write separate insert/update logic and check which one applies yourself.

Imagine importing customer records from an external CRM system nightly. Each import needs to know: "does this customer already exist in Salesforce, or is this new?" The external system's own customer ID — not Salesforce's internal Id — is what naturally identifies "the same customer" across both systems. An external Id field (a custom field marked as an External ID) lets upsert match on that value instead of Salesforce's own Id.

List<Account> importedAccounts = new List<Account>{
    new Account(Name = 'Riverbend Farms', External_Customer_Id__c = 'CRM-4471'),
    new Account(Name = 'Northwind Traders', External_Customer_Id__c = 'CRM-4472')
};

upsert importedAccounts External_Customer_Id__c;

upsert importedAccounts External_Customer_Id__c tells Salesforce to match each incoming record against existing Accounts by comparing External_Customer_Id__c, not the internal Id — a record whose External_Customer_Id__c already exists gets updated; a new one gets inserted. This is the standard, safe way to repeatedly sync data from an outside system without creating duplicates.