MySQL --- OR in a select statement ???

Is it possible to include an OR condition in a MySQL select on 2 different tables? I have 1 table of basic customers and one table of Admins, both tables "Relate" to the UserNum table containing UserNumber login password and privs, I want to select one field from the correct table based on the usernumber which is unique to all users and stored in the root User table.... trying to do something like this.... SELECT first_name FROM admin_table, user_table WHERE userNum = 1000 The info will only be in one or the other.... but what (if any) is the correct syntax?