Finding nearest neighbors between two tables with point locations in SpatiaLite?

If you don't want to compute the distances between all the point combinations, you can use a spatial index on one of the table :

SELECT 
  A.id , 
  B.myValue, 
  MIN(Distance(A.Geometry, B.Geometry)) AS distance
FROM tableOne AS A, tableTwo AS B
WHERE A.ROWID IN (
  SELECT ROWID
  FROM SpatialIndex WHERE
    f_table_name = 'A' 
    AND search_frame = BuildCircleMbr(ST_X(B.Geometry), ST_Y(B.Geometry), 10000))
GROUP BY A.id, B.myValue

I have just tested this SQL and it works:

SELECT g1.OGC_FID As id1, g2.OGC_FID As id2, MIN(ST_Distance(g1.GEOMETRY,g2.GEOMETRY)) AS DIST
FROM table_01 As g1, table_02 As g2   
WHERE g1.OGC_FID <> g2.OGC_FID
AND ST_Contains(ST_Expand(g1.geometry,50),g2.geometry)
GROUP BY id1
ORDER BY id1

As you can read here "The naive way to carry out a nearest neighbour query is to order the candidate table by distance from the query geometry, and then take the record with the smallest distance".

Best regards,

Andrea

Since version 4.4.0 SpatiaLite supports a KNN virtual table index for nearest neighbor problems. Here is a query the finds the nearest line in a linestring table to each point in a point table.

SELECT k.* FROM knn k, points p
WHERE f_table_name = 'linestrings' 
AND ref_geometry = p.geometry
AND max_items = 1;