how to add serial num in sql?

I have a tatabase table which has three columns:FistName,LastName,Grade. I would like to run a query such that a new column serves as serial number can be seen. I guess there might be a function, say, serialNumber(), that I can use in the following query:
select serialNumber(),FistName,LastName,Grade form myTable;
For some reason, it is impossible or not desired to have a column serveing as serial number built in.
Please advise. Thanks.