PHP SQL SUM column with condition

You need to GROUP BY orderdate if you want to get the sum for each date. This creates one group for each of the distinct values of orderdate, and you can then use the aggregate function SUM() to get each sum for each group (so for you, that's the sum of price for each date).

If you do not supply a GROUP BY orderdate, you just get the sum across all rows.

SELECT SUM(price) as totalprice, orderdate
FROM table_order
WHERE deptnr='$departmetnnr' 
  AND orderstatus='CLOSE' 
  AND orderdate  BETWEEN '$oneyearbefore' AND '$currentdate' 
GROUP BY orderdate
ORDER BY orderdate

That said, you are currently injecting variables directly into the query, which should be avoided by using a prepared statement.

$stmt = $con->prepare("SELECT SUM(price) as totalprice, orderdate
                       FROM table_order
                       WHERE deptnr=? 
                         AND orderstatus='CLOSE' 
                         AND orderdate  BETWEEN ? AND ? 
                       GROUP BY orderdate
                       ORDER BY orderdate");
$stmt->bind_param("sss", $departmetnnr, $oneyearbefore, $currentdate);
$stmt->execute();
$stmt->bind_result($totalprice, $orderdate);
while ($stmt->fetch()) {
    $data[] = ["totalprice" => $totalprice, "orderdate" => $orderdate];
}
$stmt->close();
$json = json_encode($data);

You can also use SQL functions to create 1 year in the past and get the current date, instead of using PHP values. Use CURDATE() to get the current date, then define an interval of 1 year which you subtract from, these will then become the ranges for your BETWEEN.

$stmt = $con->prepare("SELECT SUM(price) as totalprice, orderdate
                       FROM table_order
                       WHERE deptnr=? 
                         AND orderstatus='CLOSE' 
                         AND orderdate  BETWEEN CURDATE() AND CURDATE() - INTERVAL 1 YEAR
                       GROUP BY orderdate
                       ORDER BY orderdate");
$stmt->bind_param("s", $departmetnnr);
$stmt->execute();
$stmt->bind_result($totalprice, $orderdate);
while ($stmt->fetch()) {
    $data[] = ["totalprice" => $totalprice, "orderdate" => $orderdate];
}
$stmt->close();
$json = json_encode($data);