Add an answer to city boundaries lesson (#9427)

docs/training_manual/spatial_databases/geometry.rst

@@ -194,6 +194,67 @@ Try Yourself: :abbr:`★★★ (Advanced level)`
 Create city boundaries by computing the minimum convex hull of all addresses
 for that city and computing a buffer around that area.
 
+.. admonition:: Answer
+  :class: dropdown
+
+  - Add some people in 'Tokyo Outer Wards' city
+
+   .. code-block:: psql
+
+    INSERT INTO people (name, house_no, street_id, phone_no, city_id, geom)
+       VALUES ('Bad Aboum',
+               57,
+               2,
+               '073 712 31 21',
+               2,
+               'SRID=4326;POINT(22 18)');
+
+    INSERT INTO people (name, house_no, street_id, phone_no, city_id, geom)
+       VALUES ('Pat Atra',
+               59,
+               2,
+               '074 712 31 25',
+               2,
+               'SRID=4326;POINT(23 14)');
+
+    INSERT INTO people (name, house_no, street_id, phone_no, city_id, geom)
+       VALUES ('Kat Herin',
+               65,
+               2,
+               '074 722 31 28',
+               2,
+               'SRID=4326;POINT(29 18)');
+
+  - Create myPolygonTable table  
+
+   .. code-block:: psql
+
+    CREATE TABLE myPolygonTable (
+      id serial NOT NULL PRIMARY KEY,
+      city_id int NOT NULL REFERENCES cities(id),
+      geom geometry NOT NULL
+    );
+
+    ALTER TABLE myPolygonTable
+      ADD CONSTRAINT myPolygonTable_geom_polygon_chk
+      CHECK (st_geometrytype(geometry) = 'ST_Polygon'::text );
+
+  - Create and load the convex hulls
+
+   .. code-block:: psql
+
+    INSERT INTO myPolygonTable (city_id, geometry)
+      SELECT * FROM 
+      (
+        SELECT 
+          ROW_NUMBER() over (order by city_id)::integer AS city_id,
+          ST_CONVEXHULL(ST_COLLECT(geom)) AS geom
+            FROM people
+            GROUP BY city_id
+      ) convexHulls;
+
+
+
 
 Access Sub-Objects
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