Query spatial data¶

After you installed and set up PostGIS, let’s find answers to the following questions by querying the database:

What is the population of the New York City?¶

SELECT Sum(popn_total) AS population
  FROM nyc_census_blocks;

Output:

population
------------
    8175032
(1 row)

To get the answer we will use the ST_Area function that returns the areas of polygons.

SELECT ST_Area(geom) / 1000000
  FROM nyc_neighborhoods
  WHERE name = 'Central Park';

Output:

      st_area
--------------------
 3.5198365965413293
(1 row)

By default, the output is given in square meters. To get the value in square kilometers, divide it by 1 000 000.

SELECT ST_Length(geom)
  FROM nyc_streets
  WHERE name = 'Columbus Cir';

Output:

     st_length
-------------------
 308.3419936909855
(1 row)