Can't use a variable for a column name?

A user enters a name into a textfield and clicks on a "Find" button.  Depending on which text field the user entered the data, the appropriate column name in the table is used for fieldName and the entered text is passName.  However, the fieldName doesn't return anything.  But if I replace fieldName with the column name ("WHERE first_name='"), the program returns values.  Can't we use variables for column names or do I have to just put it all in an if/else statement?

Here is my code:

              result = fe.executeQuery("SELECT first_name, last_name, emp_nbr, emp_type_code, emp_status_code, emp_work_center " +
                                            "FROM employee " +
                                            "WHERE '"+fieldName+"'='"+passName+"'");