Finding drivers from nearest locations of a given location - mysql

Question

I have 3 tables.
Table 1 - drivers
Table 2 - location
Table 3 - distance
User will search for a driver that matches a location. In drivers table the location refers to the current location of the driver. If a driver is not available in a particular location, I want a query to search for a driver that is closest to the location that the user has provide.
Table 2 is the location names and Table 3 is the distance from one place to another. But the problem is, if distance from locationid 1 to locationid 2 is stored, opposite version is not (locationid 2 to locationid 1).

Answer 1

This is not straight forward.
First you need to get all the location with which the given location has a relationship. This result includes the locationid and distance.
The distance with the location itself is zero. I've taken help of UNION ALL in order to make a list of <locationid,distance>.
Then make an INNER JOIN between the above list and your drivers table on matching location.
And finally sort the result set based on distance in ascending order.
SELECT
*
FROM drivers DR
INNER JOIN
(
SELECT
locationid,
0 AS distance
FROM location
WHERE locationname = 'Gulshan'
UNION ALL
SELECT
IF(L.locationid = D.fromid, D.toid, D.fromid),
D.distance
FROM location L
INNER JOIN distance D ON L.locationid IN (D.fromid,D.toid)
WHERE locationname = 'Gulshan'
) AS t
ON DR.location = t.locationid
ORDER BY t.distance
See Rextester Demo
OR
See SQL Fiddle Demo
Note: You may include LIMIT n in order to restrict the result set containing at most top n search results.
You can also include a ..WHERE distance < MAX_ALLOWABLE_DISTANCE... in your query so that the final result makes some sort of sense.