Dear Birgit, Very many thanks. The query is now running – so it is no longer giving an error. I will not know for a few hours whether it is calculating the necessary values but it seems progress is being made.
I have a book coming on SQL – I clearly need to do my homework more thoroughly ☺ I will confirm when it stops! Best wishes Darrel From: postgis-users [mailto:[email protected]] On Behalf Of Birgit Laggner Sent: 24 November 2015 12:19 To: [email protected] Subject: Re: [postgis-users] Help with SQL query? Hi Darrel, my PostGIS version is too old for testing, but if I read the documentation right, then your expression has to be SQL. And IF THEN ELSE etc. is not SQL as far as I know - SQL has CASE WHEN. So, I would assume, you would need to write your expression like this: 'CASE WHEN [rast2] > 0.0 THEN [rast1] ELSE NULL END' I am curious if this helps with your error... Regards, Birgit Am 24.11.2015 um 10:09 schrieb Darrel Maddy: Dear Roxanne, Many thanks. I did pickup that issue shortly after my last post and it cured the first problem. There is still an SQL issue with the expression however. I tried ‘IF [rast2]>0.0 THEN [rast1] ELSE NULL ENDIF’ This produces an error at $2. I can remove that problem by doing this ‘IF ([rast2]>0.0) THEN [rast1] ELSE NULL ENDIF’ But then I get the error at THEN Curiously ‘[rast2]+[rast1]’ works as intended so somehow the conditional is being specified incorrectly. Unfortunately there is nothing in the documentation which helps me with this variant. I will keep trying. Darrel From: postgis-users [mailto:[email protected]] On Behalf Of Roxanne Reid-Bennett Sent: 24 November, 2015 12:46 AM To: [email protected]<mailto:[email protected]> Subject: Re: [postgis-users] Help with SQL query? On 11/23/2015 12:41 PM, Darrel Maddy wrote: Dear Pierre, I was not looking for a total solution and I am grateful for the suggestion. Although it may not look like it, I did consult the documentation and also I have Regina’s book beside me. Unfortunately, for me at least, both documents assume some knowledge of SQL – which I do not have. I am also trying to do this simultaneously with a large number of other things that are new to me. I do not find the errors reported at all informative and consider the query I am trying to perform to be relatively trivial and hence I had hoped the structure of the query might have been more intuitive. For others it may be. FWIW - I don't play with rasters, but this appears to be a pure SQL thing... add "as rast" like below and try again. SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'IF concentrated > 6 THEN deposition ELSE NULL ENDIF ' ) as rast FROM mymodel.deposition, mymodel.concentrated WHERE ST_UpperleftX(mymodel.deposition.rast) = ST_UpperleftX(mymodel.concentrated.rast) AND ST_UpperleftY(mymodel.deposition.rast) = ST_UpperleftY(mymodel.deposition.rast) Roxanne I will try not to bother you again. Darrel From: postgis-users [mailto:[email protected]] On Behalf Of Pierre Racine Sent: 23 November 2015 20:30 To: PostGIS Users Discussion <[email protected]><mailto:[email protected]> Subject: Re: [postgis-users] Help with SQL query? The expression has to stay as it was: 'IF [rast2] > threshold THEN [rast1] ELSE NULL ENDIF ' Just replace the threshold value as you did. Do not try to replace the [rast2] and [rast1]. They refer to the first and second raster pixel values. Read the ST_Mapalgebra doc… Don’t expect our suggestions to work blindly. I did not test this query. I’m not in your context. I expect you read the doc about all the mentioned functions and adjust for your specific context. I said “your query should “look like” this”… Pierre From: postgis-users [mailto:[email protected]] On Behalf Of Darrel Maddy Sent: Monday, November 23, 2015 2:47 PM To: PostGIS Users Discussion Subject: Re: [postgis-users] Help with SQL query? OK I spoke too soon. I tried this: SELECT (ST_SummaryStats(ST_Union(rast))).sum sum FROM (SELECT ST_MapAlgebra(deposition.rast, concentrated.rast, 'IF concentrated > 6 THEN deposition ELSE NULL ENDIF ' ) FROM mymodel.deposition, mymodel.concentrated WHERE ST_UpperleftX(mymodel.deposition.rast) = ST_UpperleftX(mymodel.concentrated.rast) AND ST_UpperleftY(mymodel.deposition.rast) = ST_UpperleftY(mymodel.deposition.rast) ) foo; And all I get is rast does not exist. I’m afraid the penny has not dropped yet ☹ Darrel From: postgis-users [mailto:[email protected]] On Behalf Of Darrel Maddy Sent: 23 November 2015 16:30 To: PostGIS Users Discussion <[email protected]<mailto:[email protected]>> Subject: Re: [postgis-users] Help with SQL query? Dear Pierre and Rasmus, Many thanks for trying to help. Rasmus: I am aware of GMT but I was looking for a solution in postgis so that I can keep all of the data extraction in one place. Pierre: That is exactly what I was looking for and very many thanks for including the explanation. I am a little overwhelmed with the number of functions offered in postgis. It is certainly a remarkable tool. Watching the queries plough through my datasets is a pleasure – albeit the results do not always please me ☺ Hopefully I can put this to work later tonight. Best wishes Darrel From: postgis-users [mailto:[email protected]] On Behalf Of Pierre Racine Sent: 23 November 2015 16:17 To: PostGIS Users Discussion <[email protected]<mailto:[email protected]>> Subject: Re: [postgis-users] Help with SQL query? Darrel, 1) Create a new raster selecting the right pixels with ST_MapAlgebra(raster, raster) 2) Make sure only intersecting rasters are processed by using their upper left corner X and Y coordinates (with ST_UpperLeftX() and UpperLeftY()) 3) Sum the selected pixels with ST_SummaryStats(rast) All in all a global query should look like this: SELECT (ST_SummaryStats(ST_Union(rast))).sum sum FFOM (SELECT ST_MapAlgebra(tableA.rast, tableB.rast, 'IF [rast2] > threshold THEN [rast1] ELSE NULL ENDIF ' ) FROM tableA, tableB WHERE ST_UpperleftX(tableA.rast) = ST_UpperleftX(tableB.rast) AND ST_UpperleftY(tableA.rast) = ST_UpperleftY(tableB.rast) AND maybe some other condition here if you get time series e.g. tableA.year = tableB.year AND tableA.month = tableB.month) foo If you have millions of tile you could create indexes on ST_UpperleftX(tableA.rast), ST_UpperleftX(tableB.rast), ST_UpperleftY(tableA.rast) and ST_UpperleftY(tableB.rast) to make the query faster. You could also just use WHERE ST_Intersects(tableA.rast, tableB.rast) instead… Pierre From: postgis-users [mailto:[email protected]] On Behalf Of Darrel Maddy Sent: Monday, November 23, 2015 7:20 AM To: PostGIS Users Discussion Subject: [postgis-users] Help with SQL query? Dear all, As you know I am relatively new to postgis and SQL and therefore I have much to learn. However, I am facing a paper deadline and need to do some quick analysis of the data I have and I am struggling to figure out how best to pursue what I need to do. I have a significant number of rasters which have double precision values. Without going into detail about what the rasters represent, I need to extract and sum values from one set of rasters in say table A based upon values in another set of rasters in say table B where the pixel value in the raster from Table B exceeds a threshold. Both tables are the same size (rasters are tiled) but I also need to figure out how I make sure the correct rasters are compared. They have filenames like this rastervariable_10.tif, rastervariable_100.tif , presumably I need to use a logical expression to strip the numerical value (in this case this represents the year) and then order on that basis? I can do this in QGIS one at a time but that is a little clumsy and rather time consuming. If someone can just point me in the right direction I am sure I can figure out the rest for myself. Apologies once more for asking what is probably a rather trivial question and yet again demonstrating my ignorance. Many thanks Darrel _______________________________________________ postgis-users mailing list [email protected]<mailto:[email protected]> http://lists.osgeo.org/mailman/listinfo/postgis-users -- [At other schools] I think the most common fault in general is to teach students how to pass exams instead of teaching them the science. 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