I'm using this formula to calculate the distance between entries in my (My)SQL database which have latitude and longitude fields in decimal format:
6371 * ACOS(SIN(RADIANS( %lat1% )) * SIN(RADIANS( %lat2% )) +
COS(RADIANS( %lat1% )) * COS(RADIANS( %lat2% )) * COS(RADIANS( %lon2% ) -
RADIANS( %lon1% )))
Substituting %lat1% and %lat2% appropriately it can be used in the WHERE clause to find entries within a certain radius of another entry, using it in the ORDER BY clause together with LIMIT will find the nearest x entries etc.
I'm writing this mostly as a note for myself, but improvements are always welcome. :)
Note: As mentioned by Valerion below, this calculates in kilometers. Substitute 6371 by an appropriate alternative number to use meters, miles etc.
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