How to add up the figures of the same day? Printed as correct json - Codes Helper

this is the result of the query

this is the php query code

SELECT
FROM_UNIXTIME(add_time, "%Y-%m-%d") as add_time,
count(view_id) as viewTime
FROM `view`
GROUP BY `add_time`

this is the whole php code

header("Content-type: application/json");
    $chartData = $pdo->query(
      "SELECT
FROM_UNIXTIME(add_time, "%Y-%m-%d") as add_time,
count(view_id) as viewTime
FROM `view`
GROUP BY `add_time` "
    );

    $data = array();
    $key = 0;
    foreach ($chartData as $item){
      $data[$key] = array(
        "date" => $item["add_time"],
        "number" => $item["viewTime"]
      );

      $keyPP;
    }
    echo json_encode($data);

print results

[{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"},{"date":"2018-08-16","number":"1"}.............

the viewTime on the right is all data
, but what I want to implement is
date duplicates as long as a
is shown, but the numbers of the same date are all added up
. How can this be achieved?
I"m going to print out a json like this at the back end
with a sense of principle if possible