Question

I have 2 tables:

tZipCodeNoCity with ZipCode and PointGeography
and MBLPosition with Latitude and Longitude

In this query I’m finding closest ZipCode to my positions. It’s “poor mans” geocoding 🙂

How do I write this query so I don’t have to do this SELECT TOP 1 inline?
It’s pretty slow with even 150 devices (like 20 seconds)

I had to include 150 mile radius into this subselect to get it faster

SELECT LastPositions.DeviceId, P.Description, P.Latitude, P.Longitude, P.Speed, P.DeviceTime,
(
SELECT TOP 1 ZC.ZipCode
FROM dbo.tZipCodeNoCity ZC
WHERE ZC.PointGeography.STDistance(geography::STPointFromText('POINT(' + CAST(P.Longitude AS VARCHAR(20)) + ' ' + CAST(P.Latitude AS VARCHAR(20)) + ')', 4326)) < 150 * 1609.344
ORDER BY ZC.PointGeography.STDistance(geography::STPointFromText('POINT(' + CAST(P.Longitude AS VARCHAR(20)) + ' ' + CAST(P.Latitude AS VARCHAR(20)) + ')', 4326))
) 
            FROM dbo.MBLPosition P
            INNER JOIN 
            (
                SELECT D.DeviceId, MAX(P.PositionKey) LastPositionKey 
                FROM dbo.MBLPosition P
                INNER JOIN IDATTApplication.dbo.MBLDevice D ON P.DeviceKey = D.DeviceKey
                GROUP BY D.DeviceId
            ) LastPositions ON P.PositionKey = LastPositions.LastPositionKey

Answer 1

In a project I worked on about 12 years ago, I ran a query along these lines to reduce the list of possibilities before doing the actual distance calculation:

WHERE zip.lat < my.lat + 0.5 && zip.lat > my.lat - 0.5 
    && zip.long < my.long + 0.5 && zip.long > my.long - 0.5

From that subset, I calculate the actual distance between the two points and sort on it. You’ll have to adjust the “0.5” portion as appropriate to get a big enough box to be sure you’re going to get a hit.

And I would imagine that there’s a better way than STPointFromText to create your point object. Could you use STPointFromWKB? Could you convert to the geography type once?

See this page for an example of creating your point via SET.

DECLARE @p geography;
SET @p = geography::STGeomFromText('POINT(' + CAST(P.Longitude AS VARCHAR(20)) + ' ' + CAST(P.Latitude AS VARCHAR(20)) + ')', 4326);

SELECT TOP 1 ZC.ZipCode
FROM dbo.tZipCodeNoCity ZC
WHERE ZC.PointGeography.STDistance(@p)) < 150 * 1609.344
ORDER BY ZC.PointGeography.STDistance(@p))