Simplified the SQL query and optimized for processing speed

exposurejapan/database.py

@@ -1065,16 +1065,13 @@ class JapanDatabase(SpatialiteDatabase):
         # Calculate the population_density with the boundary area from Districts table
         # and update the PopulationDistribution table
         sql_statement = (
-            "WITH updates(ID, population_density) "
-            "AS (SELECT PopulationDistribution.district_id, PopulationDistribution.total/ "
-            "District.area_size "
-            "FROM PopulationDistribution "
-            "INNER JOIN District ON District.id = PopulationDistribution.district_id) "
-        )
-        sql_statement += (
             "UPDATE PopulationDistribution "
-            "SET population_density = (SELECT population_density FROM updates "
-            "WHERE PopulationDistribution.district_id = ID)"
+            "SET population_density = Q.density FROM "
+            "(SELECT PopulationDistribution.total/District.area_size AS density, "
+            "PopulationDistribution.district_id AS id "
+            "FROM PopulationDistribution INNER JOIN District "
+            "ON District.id = PopulationDistribution.district_id) AS Q "
+            "WHERE PopulationDistribution.district_id = Q.id"
         )
         logger.debug(sql_statement)
         self.cursor.execute(sql_statement)