zipcodes within a specific radius - sql

Question

I'm looking for an SQL query to let me retrieve Zipcodes within a specific radius.
I have a table called tblZip with columns: Zipcode, Lat(latitude), Long(Longitude).
tblZip
Zipcode | Lat | Long
Short Text | Number | Number
I have checked few other answers that suggest use of Great-circle Distance but I don't seem to understand how it works. I get a headache just seeing all the conversions from / to radians and degrees and I really tried to understand it, but I'm so bad with mathematics.
I appreciate your help and guidance.
PS: I'm using a Microsoft Access Database.

Answer 1

Consider using the haversine distance formula in a VBA function (borrowed here). Then have it called in your SQL query. And even have your query filtered by distance with WHERE clause per your needs. Do note: this solution will only work inside the MSAccess.exe program. You will not be able to call it externally via ODBC/OLEDB.
And since you need comparison geocodes, SQL query below uses a cross join where each record is compared with every other record in table. Additionally, query avoids reverse duplicates and same-matched pairings reducing the size. But be careful if table is large as cross joins on self returns N2 records, that is before the filtering of duplicates.
SQL (Zipcode pairings with distance of 5 km or less)
SELECT z1.Zipcode, z2.Zipcode,
GetDistance(z1.Lat, z1.Lon, z2.Lat, z2.Lon) As km_distance
FROM tblZip z1, tblZip z2
WHERE z1.Zipcode > z2.Zipcode
AND GetDistance(s1.lat, s1.lon, s1.lat, s2.lon) <= 5;
VBA (save in a standalone module)
Function GetDistance(lat1Degrees As Double, lon1Degrees As Double, lat2Degrees As Double, lon2Degrees As Double) As Double
Dim earthSphereRadiusKilometers As Double
Dim kilometerConversionToMilesFactor As Double
Dim lat1Radians As Double
Dim lon1Radians As Double
Dim lat2Radians As Double
Dim lon2Radians As Double
Dim AsinBase As Double
Dim DerivedAsin As Double
'Mean radius of the earth (replace with 3443.89849 to get nautical miles)
earthSphereRadiusKilometers = 6371
'Convert kilometers into miles (replace 0.621371 with 1 to keep in kilometers)
kilometerConversionToMilesFactor = 0.621371
'Convert each decimal degree to radians
lat1Radians = (lat1Degrees / 180) * (4 * ATN(1))
lon1Radians = (lon1Degrees / 180) * (4 * ATN(1))
lat2Radians = (lat2Degrees / 180) * (4 * ATN(1))
lon2Radians = (lon2Degrees / 180) * (4 * ATN(1))
AsinBase = Sin(Sqr(Sin((lat1Radians - lat2Radians) / 2) ^ 2 + Cos(lat1Radians) * Cos(lat2Radians) * Sin((lon1Radians - lon2Radians) / 2) ^ 2))
DerivedAsin = (AsinBase / Sqr(-AsinBase * AsinBase + 1))
'Get distance from [lat1,lon1] to [lat2,lon2]
GetMiles = Round(2 * DerivedAsin * (earthSphereRadiusKilometers * kilometerConversionToMilesFactor), 2)
End Function