Question

I have a query that retrieves location entities within a certain radius given a latitude and longitude.

I’m now trying to adapt that query to return a list of users within a certain radius, with their most recent location alongside it. My problem is that if I GROUP BY the user id, I can’t then sort the locations to only return the most recent one.

I’ve tried following ypercube’s advice on using a subquery which has got me this far:

SET @mylon = -1.095414;
SET @mylat = 50.79486;

SELECT  u.*, dest.*
FROM    users AS u
JOIN    locations AS dest
ON      dest.id = 
        (
        SELECT  l.id, 3956 * 2 * ASIN(SQRT(POWER(SIN((@mylat - abs(l.latitude)) * pi()/180/2),
                2) + COS(@mylat * pi()/180) * COS(abs(l.latitude) *
                pi()/180) * POWER(SIN((@mylon - l.longitude) *
                pi()/180/2),2))) as distance
        FROM    locations AS l
        HAVING  distance < 0.5
        ORDER BY
                l.created DESC
        LIMIT 1
        )

However, this returns an SQL error as I’ve got more than one field being returned by my subquery.

Any ideas how to fix this? Thanks!

Answer 1

First, your subquery must return only an id that should be matched with the dest.id.
I don’t see you using GROUP BY so you should replace HAVING with WHERE.
I think you can directly say WHERE (3956 * 2 * ASIN(…….) < 0.5 so that you don’t put that computation in the result and leave only the id

SET @mylon = -1.095414;
SET @mylat = 50.79486;

SELECT  u.*, dest.*
FROM    users AS u
JOIN    locations AS dest
ON      dest.id = 
    (
    SELECT  l.id
    FROM    locations AS l
    WHERE (3956 * 2 * ASIN(SQRT(POWER(SIN((@mylat - abs(l.latitude)) * pi()/180/2),
            2) + COS(@mylat * pi()/180) * COS(abs(l.latitude) *
            pi()/180) * POWER(SIN((@mylon - l.longitude) *
            pi()/180/2),2)))) < 0.5
    ORDER BY
            l.created DESC
    LIMIT 1
    )

To return also the computation you may be able to create a temporary table like this

SET @mylon = -1.095414;
SET @mylat = 50.79486;

SELECT  u.*, dest.*
FROM    users AS u
JOIN    (
    SELECT  l.id, l.created, (3956 * 2 * ASIN(SQRT(POWER(SIN((@mylat - abs(l.latitude)) * pi()/180/2),
        2) + COS(@mylat * pi()/180) * COS(abs(l.latitude) *
        pi()/180) * POWER(SIN((@mylon - l.longitude) *
        pi()/180/2),2)))) as distance
FROM    locations  ) as location_table_tmp
ON u.id = location_table_tmp.id
ORDER BY location_table_tmp.created DESC

I’m not sure if the syntax is Ok, try to tweak a little bit and hope it works