Get 5 rows from table - php

Question

I want to fetch 5 rows from table whose "lat" value is nearest to 30. I am developing Google MAP app, where i need the 5 nearest location from Data Base.
My table looks like that,

Answer 1

MySQL provides a Math function that turns negative numbers into absolute values. By using that, you can get the five closest locations whether their lat is slightly lower or higher than 30:
ORDER BY ABS(lat - 30) ASC LIMIT 5
The ASC is optional as it is the default sorting order in all DBMS (thanks Gordon).

Answer 2

On a map "nearest location" should be based on the distance of two points, otherwise lat 30.00, long 75.00 will be the "nearest" location.
A quite exact calculation of the distance between two points (latitude/longitude) is based on the Haversine formula:
DEGREES(ACOS(COS(RADIANS(ref_latitude)) *
COS(RADIANS(latitude)) *
COS(RADIANS(ref_longitude) - RADIANS(longitude)) +
SIN(RADIANS(ref_latitude)) *
SIN(RADIANS(latitude)))) AS distance
latitude = `lat`
longitude = `long`
ref_latitude & ref_longitude = the point you want to find the nearest locations from
`DOUBLE` should be used for calculation
This results in degrees, multiply by 111.195 for an approximate distance in kilometers or by 69.093 for miles.
As you want near locations you might go for a more simple calculation using the Pythagorean theorem
sqrt(power(lat-ref_latitude, 2) +
power((lng-ref_longitude)*cos(radians(ref_latitude)), 2))
Again multiply by 111.195 for kilometers or by 69.093 for miles.
Now simply ORDER BY this distance.
And instead of comparing to all rows in your database you should restrict the number of rows to compare, e.g.
WHERE latitude BETWEEN ref_latitude - 0.2 and ref_latitude + 0.2
AND longitude BETWEEN ref_longitude - 0.2 and ref_longitude + 0.2
Btw, some DBMSes support geospatial extensions like distance functions or geospatial indexes.

Answer 3

As a note, if you want to do this efficiently and you have an index on lat, then the following more complex query should perform better:
select t.*
from ((select t.*
from table t
where lat <= 30
order by lat desc
limit 5
) union all
(select t.*
from table t
where lat > 30
order by lat asc
limit 5
)
) t
order by abs(lat - 30)
limit 5;
The two subqueries can use an index on lat to avoid sorting. The outer query is then only sorting 10 rows.