Example of a correlated query

Author: afs

discussion/correlated-query-ex1.md

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+This is an example of a LATERAL for a correlated top-k query.
+
+Find the top 2 temperature observations for each country.
+
+It is related to the ONCE algorithm in
+["deep injection" discussion document](https://github.com/w3c/sparql-query/blob/main/discussion/Defining_the_DEEP_INJECTION_approach_for_EXISTS.md)
+because it operates on each outer partial result.
+
+```
+PREFIX : <http://example/>
+
+:A :location "Country A" ;
+   :temperature 23 ;
+   :temperature 25 ;
+   :temperature 30 ;
+   :temperature 18 ;
+   .
+
+:B :location "Country B" ;
+   :temperature 13 ;
+   :temperature 15 ;
+   .
+
+:C :location "Country C" ;
+   :temperature 5 ;
+   .
+  
+:D :location "Country D" ;
+   .
+```
+
+Query:
+
+```
+PREFIX : <http://example/>
+
+## Two highest temperatures for each location
+SELECT * {
+    ?x :location ?label .
+    LATERAL {
+       SELECT * {
+         ?x :temperature ?temp
+       }
+       ORDER BY DESC(?temp)
+       LIMIT 2
+   } 
+} GROUP BY ?x
+```
+
+Results:
+
+```
+---------------------------
+| x  | label       | temp |
+===========================
+| :A | "Country A" | 30   |
+| :A | "Country A" | 25   |
+| :B | "Country B" | 15   |
+| :B | "Country B" | 13   |
+| :C | "Country C" | 5    |
+---------------------------
+```