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<p class="MsoNormal"><span style="color:#1F497D">Darrel,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Create a new raster selecting the right pixels with ST_MapAlgebra(raster, raster)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">2)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Make sure only intersecting rasters are processed by using their upper left corner X and Y coordinates (with ST_UpperLeftX() and UpperLeftY())<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">3)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Sum the selected pixels with ST_SummaryStats(rast)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">All in all a global query should look like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">SELECT (ST_SummaryStats(ST_Union(rast))).sum sum<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">FFOM (SELECT ST_MapAlgebra(tableA.rast, tableB.rast, 'IF [rast2] > threshold THEN [rast1] ELSE NULL</span> ENDIF
<span style="color:#1F497D">' )<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> FROM tableA, tableB<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> WHERE ST_UpperleftX(tableA.rast) = ST_UpperleftX(tableB.rast) AND
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> ST_UpperleftY(tableA.rast) = ST_UpperleftY(tableB.rast) AND
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> maybe some other condition here if you get time series e.g. tableA.year = tableB.year AND tableA.month = tableB.month) foo<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">You could also just use WHERE ST_Intersects(tableA.rast, tableB.rast) instead…<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Pierre<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> postgis-users [mailto:postgis-users-bounces@lists.osgeo.org]
<b>On Behalf Of </b>Darrel Maddy<br>
<b>Sent:</b> Monday, November 23, 2015 7:20 AM<br>
<b>To:</b> PostGIS Users Discussion<br>
<b>Subject:</b> [postgis-users] Help with SQL query?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-GB">Dear all,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">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?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">I can do this in QGIS one at a time but that is a little clumsy and rather time consuming.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">If someone can just point me in the right direction I am sure I can figure out the rest for myself.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Apologies once more for asking what is probably a rather trivial question and yet again demonstrating my ignorance.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Many thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Darrel<o:p></o:p></span></p>
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