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source: trunk/workshop-foss4g/joins_exercises.rst @ 16

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[1]1.. _joins_exercises:
2
3Section 13: Spatial Joins Exercises
4===================================
5
6Here's a reminder of some of the functions we have seen.  Hint: they should be useful for the exercises!
7
8* :command:`sum(expression)`: aggregate to return a sum for a set of records
9* :command:`count(expression)`: aggregate to return the size of a set of records
10* :command:`ST_Area(geometry)` returns the area of the polygons
11* :command:`ST_AsText(geometry)` returns WKT ``text``
12* :command:`ST_Contains(geometry A, geometry B)` returns the true if geometry A contains geometry B
13* :command:`ST_Distance(geometry A, geometry B)` returns the minimum distance between geometry A and geometry B
14* :command:`ST_DWithin(geometry A, geometry B, radius)` returns the true if geometry A is radius distance or less from geometry B
15* :command:`ST_GeomFromText(text)` returns ``geometry``
16* :command:`ST_Intersects(geometry A, geometry B)` returns the true if geometry A intersects geometry B
17* :command:`ST_Length(linestring)` returns the length of the linestring
18* :command:`ST_Touches(geometry A, geometry B)` returns the true if the boundary of geometry A touches geometry B
19* :command:`ST_Within(geometry A, geometry B)` returns the true if geometry A is within geometry B
20 
21Also remember the tables we have available:
22
23 * ``nyc_census_blocks`` 
24 
25   * name, popn_total, boroname, the_geom
26 
27 * ``nyc_streets``
28 
29   * name, type, the_geom
30   
31 * ``nyc_subway_stations``
32 
33   * name, routes, the_geom
34 
35 * ``nyc_neighborhoods``
36 
37   * name, boroname, the_geom
38
39Exercises
40---------
41
42 * **"What subway station is in 'Little Italy'? What subway route is it on?"**
43 
44   .. code-block:: sql
45 
46     SELECT s.name, s.routes
47     FROM nyc_subway_stations AS s
48     JOIN nyc_neighborhoods AS n
49     ON ST_Contains(n.the_geom, s.the_geom) 
50     WHERE n.name = 'Little Italy';
51
52   :: 
53 
54       name    | routes
55    -----------+--------
56     Spring St | 6
57     
58 * **"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')
59 
60   .. code-block:: sql
61 
62    SELECT DISTINCT n.name, n.boroname
63    FROM nyc_subway_stations AS s
64    JOIN nyc_neighborhoods AS n
65    ON ST_Contains(n.the_geom, s.the_geom) 
66    WHERE strpos(s.routes,'6') > 0;
67   
68   ::
69 
70            name        | boroname 
71    --------------------+-----------
72     Midtown            | Manhattan
73     Hunts Point        | The Bronx
74     Gramercy           | Manhattan
75     Little Italy       | Manhattan
76     Financial District | Manhattan
77     South Bronx        | The Bronx
78     Yorkville          | Manhattan
79     Murray Hill        | Manhattan
80     Mott Haven         | The Bronx
81     Upper East Side    | Manhattan
82     Chinatown          | Manhattan
83     East Harlem        | Manhattan
84     Greenwich Village  | Manhattan
85     Parkchester        | The Bronx
86     Soundview          | The Bronx
87
88   .. note::
89 
90     We used the ``DISTINCT`` keyword to remove duplicate values from our result set where there were more than one subway station in a neighborhood.
91   
92 * **"After 9/11, the 'Battery Park' neighborhood was off limits for several days. How many people had to be evacuated?"**
93 
94   .. code-block:: sql
95 
96     SELECT Sum(popn_total)
97     FROM nyc_neighborhoods AS n
98     JOIN nyc_census_blocks AS c
99     ON ST_Intersects(n.the_geom, c.the_geom) 
100     WHERE n.name = 'Battery Park';
101   
102   :: 
103
104     9928
105   
106 * **"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.)
107 
108   .. code-block:: sql
109   
110     SELECT
111       n.name,
112       Sum(c.popn_total) / (ST_Area(n.the_geom) / 1000000.0) AS popn_per_sqkm
113     FROM nyc_census_blocks AS c
114     JOIN nyc_neighborhoods AS n
115     ON ST_Intersects(c.the_geom, n.the_geom)
116     WHERE n.name = 'Upper West Side'
117     OR n.name = 'Upper East Side'
118     GROUP BY n.name, n.the_geom;
119     
120   ::
121   
122           name       |  popn_per_sqkm   
123     -----------------+------------------
124      Upper East Side | 47943.3590089405
125      Upper West Side | 39729.5779474286
126
127     
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