ORA-00904 error while querying the Oracle database table I am newbie with oracle DB and I need to List the Number of departments per region in the region table.

This is my SQL statement:


    select r.region_id, count(d.department_id) "Number of Department"
    from region r, department d
    where r.region_id = d.region_id
    group by r.region_id


I am getting this error:


  ORA-00904: "D"."region_id": invalid identifier


How can I correct this error?
When ORA-00904: invalid identifier occurs, we must enter a valid column name as it is either missing or the one entered is invalid. This error most commonly happens when we are referencing an invalid alias in a select statement.  Oracle's voice on ORA-00904 error:


  ORA-00904 string: invalid identifier
  
  Cause: The column name entered is either missing or invalid.
  
  Action: Enter a valid column name. A valid column name must begin with a letter, be less than or equal to 30 characters, and consist of only alphanumeric characters and the special characters $, _, and #. 
  If it contains other characters, then it must be enclosed in double quotation marks. It may not be a reserved word.


To avoid ORA-00904, column names must
begin with a letter.
consist only of alphanumeric and the special characters ($_#); other characters need double quotation marks around them.
be less than or equal to thirty characters.