SQL Server rand() aggregate

Question

Problem: a table of coordinate lat/lngs. Two rows can potentially have the same coordinate. We want a query that returns a set of rows with unique coordinates (within the returned set). Note that distinct is not usable because I need to return the id column which is, by definition, distinct. This sort of works (@maxcount is the number of rows we need, intid is a unique int id column):

select top (@maxcount) max(intid)
from Documents d
group by d.geoLng, d.geoLat

It will always return the same row for a given coordinate unfortunately, which is bit of a shame for my use. If only we had a rand() aggregate we could use instead of max()... Note that you can't use max() with guids created by newid().

Any ideas? (there's some more background here, if you're interested: http://www.itu.dk/~friism/blog/?p=121)

UPDATE: Full solution here

Answer 1

You might be able to use a CTE for this with the ROW_NUMBER function across lat and long and then use rand() against that. Something like:

WITH cte AS
(
    SELECT
        intID,
        ROW_NUMBER() OVER
            (
                PARTITION BY geoLat, geoLng
                ORDER BY NEWID()
            ) AS row_num,
        COUNT(intID) OVER (PARTITION BY geoLat, geoLng) AS TotalCount
    FROM
        dbo.Documents
)
SELECT TOP (@maxcount)
    intID, RAND(intID)
FROM
    cte
WHERE
    row_num = 1 + FLOOR(RAND() * TotalCount)

This will always return the first sets of lat and lngs and I haven't been able to make the order random. Maybe someone can continue on with this approach. It will give you a random row within the matching lat and lng combinations though.

If I have more time later I'll try to get around that last obstacle.

Answer 2

this doesn't work for you?

select top (@maxcount) *
from 
(
    select max(intid) as id from Documents d group by d.geoLng, d.geoLat
) t 
order by newid()

Answer 3

Where did you get the idea that DISTINCT only works on one column? Anyway, you could also use a GROUP BY clause.