为表上的每个组选择最后一个时间戳（记录）

Question

I've passed through some questions here in Stack Overflow, but after some tests it's was not possible to solve my problem. I have a table like below.

  ID           log_device_id           device_timestamp
=========================================================
  1456              0056              2019-05-19 11:45:18
  1451              0061              2019-05-19 22:00:03
  1450              0035              2019-05-19 17:16:23
  1449              0011              2019-05-19 12:31:34
  1448              0124              2019-05-18 09:11:27

I'm using the code below to get the Device ID list and timestamp;

sql ='SELECT log_device_id,max(device_timestamp) FROM devicetable group by log_device_id';
    $result = mysqli_query($conection, $sql);
         if (mysqli_num_rows($result) > 0) {
            while($row = mysqli_fetch_assoc($result)) {
            echo $row['log_device_id'];
            echo $row['device_timestamp'];
            }

The problem is, the result of this query returns the ID and Last Timestamp, but the $row['device_timestamp'] , comes empty and shows a 500 error.

The intention of this code is to check every hour the last timestamp of each device and check if it's is offline.

Thank you.

Answer 1

Here's one idea:

sql ='SELECT log_device_id,max(device_timestamp) as max_device_timestamp FROM devicetable group by log_device_id';
$result = mysqli_query($conection, $sql);
     if (mysqli_num_rows($result) > 0) {
        while($row = mysqli_fetch_assoc($result)) {
        echo $row['log_device_id'];
        echo $row['max_device_timestamp'];
        }

You set alias for max(device_timestamp) then you get value of it

Answer 2

$sql ='SELECT log_device_id, device_timestamp FROM devicetable WHERE device_timestamp = (SELECT MAX(device_timestamp) FROM devicetable) group by log_device_id';
$result = mysqli_query($conection, $sql);
if (mysqli_num_rows($result) > 0) {
    while($row = mysqli_fetch_assoc($result)) {
        echo $row['log_device_id'];
        echo $row['device_timestamp'];
    }
}

Hopefully this will work for you. Thank You.