Selecting next related row in MySQL - mysql

Question

I have a spatial dataset in MySQL 5.7 where I have the columns: id, deviceid, latlng_point, time. latlng_point is a geospatial Point.
What I'm trying to achieve is calculating distance from points. I'm unsure on how to approach this.
SELECT
ST_DISTANCE_SPHERE(latlng_point, i want the next latlng_point here) AS distance
FROM points
WHERE deviceid = 1
ORDER BY time DESC;
In PHP I would do something like this:
<?php
$conn = new mysqli($host,$user,$pass,$db);
$query = "SELECT latlng_point FROM points WHERE deviceid = 1...";
$latlng_array = array();
if ($result = $conn->query($query)) {
while ($row = $result->fetch_assoc()) {
$latlng_array[] = $row;
}
}
$distance = 0;
for ($i = 0; $i < count($latlng_array) - 1; $i++) {
$pt1 = $latlng_array[$i]['latlng_point'];
$pt2 = $latlng_array[$i+1]['latlng_point'];
$distance += haversine_function($pt1,$pt2);
}
echo "Distance: {$distance}";
?>
I'm trying to achieve something similar purely in MySQL.

Answer 1

Try this one:
SELECT SUM(ST_DISTANCE_SPHERE(p1.latlng_point, p2.latlng_point)) AS total_distance
FROM points p1
JOIN points p2 ON p2.id = (
SELECT p.id
FROM points p
WHERE p.deviceid = p1.deviceid
AND p.time > p1.time
ORDER BY p.time ASC
LIMIT 1
)
WHERE p1.deviceid = 1
The (correlated) subquery should return the id of the next point (sorted by time).
I can't tell you if it is really efficient or if it even works at all (can't test it).
However you should have an index on (deviceid, time) - Assuming that id is the primary key.