Lets take another break from my textbook, and focus on a classic GIS operation. If you are like me, you learned GIS with ARC/INFO. I started with ARC/INFO 3.2 on a DEC VAX PDP 11. Actually, my first ESRI product was AUTOMAP II on a Burroughs mainframe with the CANDE operating system – we even had to use a SYMAP ruler to plot our points for digitizing.
Anyway, for those of you who used ARC/INFO, you probably found that the NEAR function was one of your best friends. I used it every week to transfer attributes from points to lines, and vice-verse. This is pretty easy to do in SQL, but I will step through it for you to make it more understandable. Lets assume we have two vector drawings: Points and Lines. Obviously, the Points drawing has points in it, and the Lines drawing has lines in it. The SQL to replicate the NEAR function is:
1. SELECT min(dist), first(lines.id),points.[ID] FROM
2. (SELECT lines.id, points.id,distance(points.[Geom (I)],lines.[Geom (I)]) AS dist
3. FROM points, lines
4. ORDER BY dist)
5. GROUP BY points.id
We will actually start our review of the query at lines 2 through 4. This part of the query selects the line IDs and the distance between them – we will call the result of the distance query dist. Now, this part of the query selects the distance between every point and every other point, resulting in a table that is NxN in size. Line 4 helps us toward our NEAR function result by ordering the data by the distance. Therefore, the closest point-to-line pairs are now at the beginning of the results of the query.
Lines 1 and 5 work together as an aggregate query. Here we are returning the min distance. Now, what is the min distance? Its the shortest distance between the points and the line – remember that ORDER BY we used. Line 5 is really where the magic happens because there are lots of minimum distances in the query result. But, the GROUP BY clause aggregates the data by the ID for each point. Therefore, it returns the closest line ID and the distance to each point. The GROUP BY statement turns our NxN result into an N result where N represents each of the records in the Points drawing.
Once again, I hope you can see how easy it is to replicate standard GIS functions like NEAR. Sometime in the future, I will show you Intersect, Union, and Identity. But, lets get through a few more examples from my book.
Would one of you be so kind as to ask ESRI to implement spatial SQL into their next release of Arcinfo?