在AVG子查询上方，没有返回结果

Question

I am trying to get a list of students who have above average on the final exam

I first select to get average

SELECT w.LAST_NAME , AVG(s.NUMERIC_GRADE) AS NUMERIC_GRADE 
GRADE s , SECTION z, STUDENT w
WHERE s.SECTION_ID = z.SECTION_ID AND s.STUDENT_ID = w.STUDENT_ID
AND s.SECTION_ID = 90 AND s.GRADE_TYPE_CODE = 'FI'
GROUP BY w.LAST_NAME,s.NUMERIC_GRADE

and I get these four results

LAST_NAME                 NUMERIC_GRADE
------------------------- -------------
Mulroy                               83 
Da Silva                             92 
Lopez                                91 
Abid                                 84 

But when I try to get the above average from these four I get no rows, and it looks like sub-query and main-query have the same conditions. I am not sure how to do above after avg.

SELECT n.LAST_NAME , m.NUMERIC_GRADE 
FROM GRADE m , STUDENT n
WHERE m.STUDENT_ID = n.STUDENT_ID
GROUP BY n.LAST_NAME , m.NUMERIC_GRADE
HAVING COUNT(*) >
(SELECT AVG (NUMERIC_GRADE)
FROM
(SELECT w.LAST_NAME , AVG(s.NUMERIC_GRADE) AS NUMERIC_GRADE 
FROM GRADE s , SECTION z, STUDENT w
WHERE s.SECTION_ID = z.SECTION_ID AND s.STUDENT_ID = w.STUDENT_ID
AND s.SECTION_ID = 90 AND s.GRADE_TYPE_CODE = 'FI'
GROUP BY w.LAST_NAME,s.NUMERIC_GRADE))

ORDER BY n.LAST_NAME;

I want to get numberic_grade 91 and 92 because it is above average. Why is it giving me no rows when I am trying to select those who have above average on the final exam ?

Answer 1

There are several problems with your query:

• You need to use AVG in the first GROUP BY query as well
• You need to restrict the top query by the same condition as the inner query (ie grade type code and section id)
• You do not need to join section, because you are restricting on its ID, which is available from the GRADE table
• You should use aliases that at least slightly resemble the table name: it goes long way to helping readability
• You should use ANSI joins for better readability

Try a query with these corrections:

SELECT n.LAST_NAME , AVG(m.NUMERIC_GRADE)
FROM GRADE g
JOIN STUDENT s ON g.STUDENT_ID = s.STUDENT_ID -- Use ANSI joins
WHERE g.SECTION_ID = 90 AND g.GRADE_TYPE_CODE = 'FI'
GROUP BY s.LAST_NAME
HAVING AVG(g.NUMERIC_GRADE) >
   (SELECT AVG(NUMERIC_GRADE)
      FROM (
        SELECT AVG(g.NUMERIC_GRADE) AS NUMERIC_GRADE 
        FROM GRADE g
        JOIN STUDENT s ON s.STUDENT_ID = g.STUDENT_ID
        WHERE g.SECTION_ID = 90 AND g.GRADE_TYPE_CODE = 'FI'
        GROUP BY s.LAST_NAME
      )
   )
ORDER BY s.LAST_NAME;