.. _joins_exercises:

Section 13: Spatial Joins Exercises
===================================

Here's a reminder of some of the functions we have seen.  Hint: they should be useful for the exercises!

* :command:`sum(expression)`: aggregate to return a sum for a set of records
* :command:`count(expression)`: aggregate to return the size of a set of records
* :command:`ST_Area(geometry)` returns the area of the polygons
* :command:`ST_AsText(geometry)` returns WKT ``text``
* :command:`ST_Contains(geometry A, geometry B)` returns the true if geometry A contains geometry B 
* :command:`ST_Distance(geometry A, geometry B)` returns the minimum distance between geometry A and geometry B
* :command:`ST_DWithin(geometry A, geometry B, radius)` returns the true if geometry A is radius distance or less from geometry B
* :command:`ST_GeomFromText(text)` returns ``geometry``
* :command:`ST_Intersects(geometry A, geometry B)` returns the true if geometry A intersects geometry B
* :command:`ST_Length(linestring)` returns the length of the linestring
* :command:`ST_Touches(geometry A, geometry B)` returns the true if the boundary of geometry A touches geometry B
* :command:`ST_Within(geometry A, geometry B)` returns the true if geometry A is within geometry B
 
Also remember the tables we have available:

 * ``nyc_census_blocks`` 
 
   * name, popn_total, boroname, the_geom
 
 * ``nyc_streets``
 
   * name, type, the_geom
   
 * ``nyc_subway_stations``
 
   * name, routes, the_geom
 
 * ``nyc_neighborhoods``
 
   * name, boroname, the_geom

Exercises
---------

 * **"What subway station is in 'Little Italy'? What subway route is it on?"**
 
   .. code-block:: sql
 
     SELECT s.name, s.routes 
     FROM nyc_subway_stations AS s
     JOIN nyc_neighborhoods AS n 
     ON ST_Contains(n.the_geom, s.the_geom)  
     WHERE n.name = 'Little Italy';

   :: 
  
       name    | routes 
    -----------+--------
     Spring St | 6
     
 * **"What are all the neighborhoods served by the 6-train?"** (Hint: The ``routes`` column in the ``nyc_subway_stations`` table has values like 'B,D,6,V' and 'C,6')
 
   .. code-block:: sql
  
    SELECT DISTINCT n.name, n.boroname 
    FROM nyc_subway_stations AS s
    JOIN nyc_neighborhoods AS n 
    ON ST_Contains(n.the_geom, s.the_geom)  
    WHERE strpos(s.routes,'6') > 0;
    
   ::
  
            name        | boroname  
    --------------------+-----------
     Midtown            | Manhattan
     Hunts Point        | The Bronx
     Gramercy           | Manhattan
     Little Italy       | Manhattan
     Financial District | Manhattan
     South Bronx        | The Bronx
     Yorkville          | Manhattan
     Murray Hill        | Manhattan
     Mott Haven         | The Bronx
     Upper East Side    | Manhattan
     Chinatown          | Manhattan
     East Harlem        | Manhattan
     Greenwich Village  | Manhattan
     Parkchester        | The Bronx
     Soundview          | The Bronx

   .. note::
  
     We used the ``DISTINCT`` keyword to remove duplicate values from our result set where there were more than one subway station in a neighborhood.
    
 * **"After 9/11, the 'Battery Park' neighborhood was off limits for several days. How many people had to be evacuated?"**
 
   .. code-block:: sql
 
     SELECT Sum(popn_total)
     FROM nyc_neighborhoods AS n
     JOIN nyc_census_blocks AS c 
     ON ST_Intersects(n.the_geom, c.the_geom)  
     WHERE n.name = 'Battery Park';
   
   :: 

     9928
    
 * **"What are the population density (people / km^2) of the 'Upper West Side' and 'Upper East Side'?"** (Hint: There are 1000000 m^2 in one km^2.)
 
   .. code-block:: sql
   
     SELECT 
       n.name, 
       Sum(c.popn_total) / (ST_Area(n.the_geom) / 1000000.0) AS popn_per_sqkm
     FROM nyc_census_blocks AS c
     JOIN nyc_neighborhoods AS n
     ON ST_Intersects(c.the_geom, n.the_geom)
     WHERE n.name = 'Upper West Side'
     OR n.name = 'Upper East Side'
     GROUP BY n.name, n.the_geom;
     
   ::
   
           name       |  popn_per_sqkm   
     -----------------+------------------
      Upper East Side | 47943.3590089405
      Upper West Side | 39729.5779474286