return 1 formatted result instead of multiple results

I want to query a column in a table that returns multiple records,
append the values of that column in a comma separated string, and
return that string.

example:  select * from people;
returns 3 results:
  Mike
  John
  Dennis

and I want one result to be "Mike,John,Dennis".

How can I format my results in this way using SQL (preferably not
stored functions)?

Thank you.