No result found in query - php

Question

I'm getting an error in my query, and I'm unable to detect the source of the problem.
Here is the query:-
$query = "SELECT useraccount.Username, tariff.Name as tariffs,
sum(energyconsumption.ElecEnergy)
FROM useraccount
INNER JOIN tariff
ON useraccount.tariffs = tariff.id
INNER JOIN energyconsumption
ON energyconsumption.User = useraccount.id
WHERE Date = CURRENT_DATE
GROUP BY useraccount.Username, tariff.Name as tariffs";
Following the query I've code that stores the output in an array:
$result = mysqli_query($conn,$query);
$r = array();
if($result->num_rows){
while($row = mysqli_fetch_array($result)){
array_push($r, array( 'Username' => $row['Username'],
'TariffName' => $row['tariffs'], 'ElecConsump' => $row['ElecEnergy']
));
}
} echo json_encode(array('results' => $r));
Im getting an error in the following line: if($result->num_rows)
This is the output when executing the query:
Notice: Trying to get property of non-object in C:\xampp\htdocs\Project\Client\newone.php on line 22
{"results":[]}
Please note:
This was the output i intially had:
{"results":[{"Username":"absc868","TariffName":"s1","ElecConsump":"2000"},
{"Username":"absc868","TariffName":"s1","ElecConsump":"1900"}]}
But with this new query I have written above, I am trying to produce this output
I am trying to produce the following output:
= {"results":[{"Username":"absc868","TariffName":"s1","ElecConsump":"3900"}
That being, a result set that only has 1 entry, for username, tariff and elecconsump, rather than 2 entries for username, tariff and elecconsump
Thank you once again to all those who have read and contributed to this thread

Answer 1

One problem is the as in the GROUP BY. I would recommend that you use table aliases:
SELECT ua.Username, t.Name as tariffs,
SUM(ec.ElecEnergy) as ElecEnergy
FROM useraccount ua INNER JOIN
tariff t
ON ua.tariffs = t.id INNER JOIN
energyconsumption ec
ON ec.User = ua.id
WHERE Date = CURRENT_DATE
GROUP BY ua.Username, t.Name;