What happens when a comma is not placed following an existing field name in SQL

This is with regard to the SQL Tryit Editor provided by W3 Schools. (Saying this just in case it is an environment-specific problem)

The code SELECT CustomerName, Address, City, PostalCode FROM Customers; yields the following output,

While code SELECT CustomerName, Address City, PostalCode FROM Customers; is executed, I get the following result

Clearly, the missing comma has resulted in not displaying the Address field. But this is distinct from the error message that otherwise comes when a field name is either not correct or not correctly separated. It seems to recognize Address to be an existing field, while it considers to display only the field immediately before the comma.

What is the logic behind the correct use of commas? Does that mean any number of correctly named fields, but without a follow up comma, will be ignored?

Answer

Commas are used to separate the fields, when you miss a comma like you have, the 2nd column will be considered as the column alias. Here Address City means, pick the column Address and name it City. You can see in your 2nd screenshot, values from Address column are picked but the name of the column is City.