SQL LEFT JOIN WHERE not displaying right result

Question

So I got this query:

Data structure: Users id---inlog----name----more stuff

llntoets id---code----inlog----more stuff

oefeningen id---speler---status----morestuff

(inlog and speler are always the same values for a user)

SELECT
// Some other stuff working
SUM(o.status) AS oefn
FROM    users AS u
LEFT JOIN     llntoets AS l
ON (u.inlog = l.inlog)
LEFT JOIN     oefeningen AS o   
ON (u.inlog = o.speler) AND o.status = 'afgewerkt'
WHERE
code = '$code'
GROUP BY l.inlog
ORDER BY klas ASC, klasnr ASC

Everything runs fine except 1 thing the oefn variable. It shows a number sometimes it shows the correct value and sometimes it shows a value that is much higher than it should be. Someone told me it could be because of the GROUP BY. Can someone help me pls?

It is supposed to count the total records from table oefeningen where status = 'afgewerkt' and where the speler is the inlog from users. Thanks, if you got other questions ask will try to explain more.

Answer 1

the SUM(o.status) in your query it is not supposed to count the total records of table oefeningen .
that sum is the sum of the values of all the joined rows that satisfy your criteria that can be a much higher number.
also note that applying the filter o.status = 'afgewerkt' you are performing a JOIN even if you wrote LEFT JOIN throghout the query.