Although PostGIS is one of the state-of-the-art GIS softwares for effective spatial queries, aggregating effectively can sometimes be tricky. For this reason, we will go step by step through appealing to the final criterion. First of all, we need to select the markets from our POI table. This should be easy, as we just have to chain some queries on a single column together with the logical OR operator. Or we can use a more convenient operator created for similar tasks called IN. By using IN, we can supply a collection of values to check a single column against:
SELECT geom FROM spatial.pois p WHERE p.fclass IN ('supermarket', 'convenience', 'mall', 'general');
Let's put this table in a WITH clause, and go on with ...
