PHP/mysql problem with output of query

Question

I have this table in my database:

classes

id | classname | 


grades 

id | class_id | grade | date

I need to get the average of the grades and the latest grade based on the date + the correct classname.

I have this query but this doesn't output the correct classnames...

SELECT 
                AVG(grade),
                classes.classname,
                grades.grade,
                MIN(ABS(DATEDIFF(CURDATE(),'date')))
                FROM 
                grades, classes

                GROUP BY
                grades.grade_id

                ORDER BY grades.date DESC

Answer 1

Your question seemed a little vauge, so Im answering my interpretation of your question.

You didnt demonstrate what you got, but, Im going to guess you got a list of grades with an average by class, not classes with grades..

So, while I think theres a few questions in there. First.

select avg(grade), classname from classes 
left join grade on classes.id = grades.class_id 
group by classname

Should give you the average grade by class.

What is cijfers? you didnt really include much about that yet you also asked it to average it somehow.

Answer 2

First off, AVG(cijfer) is actually referencing nothing. You need to assign the AVG() to a variable like this:

AVG(grade) AS avgrade

Perhaps you could try a query like this:

SELECT AVG(grade) as average FROM grades, class_id AS cid, (SELECT classname FROM classes WHERE id=cid) AS cname FROM grades ORDER BY date DESC GROUP BY cname;

Hows that work out?

Answer 3

As much as I hate doing someone's homework for them, This seems to work for me:

select a.classname, AVG(b.grade) AS "Avg Grade",
       c.grade as "Most Recent Grade", c.grade_date as "Most Recent Grade Date"
from classes a, grades b, grades c
where a.id = b.class_id
AND c.class_id = a.id
AND NOT EXISTS
    (SELECT d.grade_date FROM grades d
     WHERE d.class_id = c.class_id
     AND   d.grade_date > c.grade_date)
group by a.classname, c.grade, c.grade_date;