Mysql Great Circle Distance

great_circle.sql

# Here's the SQL statement that will find the closest 20 locations that are within a radius of 25 miles 
# to the 37, -122 coordinate. It #calculates the distance based on the latitude/longitude of that row 
# and the target latitude/longitude, and then asks for only rows #where the distance value is less than 
# 25, orders the whole query by distance, and limits it to 20 results. To search by kilometers #instead 
# of miles, replace 3959 with 6371.

SELECT id, ( 3959 * acos( cos( radians(37) ) * 
                     cos( radians( lat ) ) *
                     cos( radians( lng ) - radians(-122) ) +
                     sin( radians(37) ) * sin( radians( lat ) ) ) ) AS distance   
                     FROM markers HAVING distance < 25
                     ORDER BY distance LIMIT 0 , 20;

#To search by kilometers instead of miles, replace 3959 with 6371.
#[ref](https://developers.google.com/maps/articles/phpsqlsearch_v3)