below AVG sub-query not showing correct result

Question

I am trying to list students who received a below average grade on the 3rd quiz.

first I select

SELECT s.STUDENT_ID, s.LAST_NAME,g.GRADE_TYPE_CODE,AVG (g.NUMERIC_GRADE) AS GRADE
FROM STUDENT s, GRADE g
WHERE s.STUDENT_ID = g.STUDENT_ID
AND g.SECTION_ID = 135 AND g.GRADE_TYPE_CODE= 'QZ' AND g.GRADE_CODE_OCCURRENCE = 3 
GROUP BY s.STUDENT_ID, s.LAST_NAME,g.GRADE_TYPE_CODE

and I get four results that received average

STUDENT_ID LAST_NAME                      GRADE
---------- ------------------------- ----------
   178      Kurtz                             98 
   215      Chatman                           90 
   259      Merriman                          81 
   214      Williams                          99 

but when I want to select those who did receive below average I get 165 rows selected

  SELECT z.STUDENT_ID, z.LAST_NAME
  FROM STUDENT z, GRADE w
  WHERE z.STUDENT_ID = w.STUDENT_ID
  GROUP BY z.STUDENT_ID, z.LAST_NAME
  HAVING COUNT(*) < 
  (SELECT AVG(GRADE) 
  FROM
  (SELECT s.STUDENT_ID, s.LAST_NAME,g.GRADE_TYPE_CODE,AVG (g.NUMERIC_GRADE) AS GRADE
  FROM STUDENT s, GRADE g
  WHERE s.STUDENT_ID = g.STUDENT_ID
  AND g.SECTION_ID = 135 AND g.GRADE_TYPE_CODE= 'QZ' AND g.GRADE_CODE_OCCURRENCE = 3 
  GROUP BY s.STUDENT_ID, s.LAST_NAME,g.GRADE_TYPE_CODE ))

 ORDER BY z.LAST_NAME;

What am I doing wrong and how can I list students who received a below average grade ?

Answer 1

If I understand your question correctly, here is a simplified version using a common table expression utilizing avg() over() :

with cte as (
  select s.student_id, s.last_name, g.numeric_grade,
    avg(g.numeric_grade) over () average
  from student s
    join grade g on s.student_id = g.student_id
  where g.section_id = 135 
    and g.grade_type_code = 'QZ' 
    and g.grade_code_occurence = 3
  )
select student_id, last_name
from cte
group by student_id, last_name
having avg(numeric_grade) < avg(average)

• Condensed SQL Fiddle Demo