I think your question says you have the
city values for the two cities between which you wish to compute the distance.
This query will do the job for you, yielding the distance in km. It uses the spherical cosine law formula.
Notice that you join the table to itself so you can retrieve two coordinate pairs for the computation.
SELECT a.city AS from_city, b.city AS to_city, 111.111 * DEGREES(ACOS(LEAST(1.0, COS(RADIANS(a.Latitude)) * COS(RADIANS(b.Latitude)) * COS(RADIANS(a.Longitude - b.Longitude)) + SIN(RADIANS(a.Latitude)) * SIN(RADIANS(b.Latitude))))) AS distance_in_km FROM city AS a JOIN city AS b ON a.id <> b.id WHERE a.city = 3 AND b.city = 7
Notice that the constant
111.1111 is the number of kilometres per degree of latitude, based on the old Napoleonic definition of the metre as one ten-thousandth of the distance from the equator to the pole. That definition is close enough for location-finder work.
If you want statute miles instead of kilometres, use
If you’re looking for nearby points you may be tempted to use a clause something like this:
HAVING distance_in_km < 10.0 /* slow ! */ ORDER BY distance_in_km DESC
That is (as we say near Boston MA USA) wicked slow.
In that case you need to use a bounding box computation. See this writeup about how to do that. http://www.plumislandmedia.net/mysql/haversine-mysql-nearest-loc/
The formula contains a
LEAST() function. Why? Because the
ACOS() function throws an error if its argument is even slightly greater than 1. When the two points in question are very close together, the expression with the
SIN() computations can sometimes yield a value slightly greater than 1 due to floating-point epsilon (inaccuracy). The
LEAST(1.0, dirty-great-expression) call copes with that problem.
Edit 2022 (by Alexio Vay):
As of today the modern solution should be the following short code:
select ST_Distance_Sphere( point(-87.6770458, 41.9631174), point(-73.9898293, 40.7628267))
Please check out the answer of Naresh Kumar.