source: trunk/workshop-foss4g/simple_sql_exercises.rst @ 10

.. _simple_sql_exercises:

Section 7: Simple SQL Exercises
===============================

Using the ``nyc_census_blocks`` table, answer the following questions (don't peak at the answers!). 

Here is some helpful information to get started.  Recall from the :ref:`About Our Data <about_data>` section our ``nyc_census_blocks`` table definition.

.. list-table::
   :widths: 20 80

   * - **blkid**
     - A 15-digit code that uniquely identifies every census **block**. Eg: 360050001009000
   * - **popn_total**
     - Total number of people in the census block
   * - **popn_white**
     - Number of people self-identifying as "white" in the block
   * - **popn_black**
     - Number of people self-identifying as "black" in the block
   * - **popn_nativ**
     - Number of people self-identifying as "native american" in the block
   * - **popn_asian**
     - Number of people self-identifying as "asias" in the block
   * - **popn_other**
     - Number of people self-identifying with other categories in the block
   * - **hous_total**
     - Number of housing units in the block
   * - **hous_own**
     - Number of owner-occupied housing units in the block
   * - **hous_rent**
     - Number of renter-occupied housing units in the block
   * - **boroname**
     - Name of the New York borough. Manhattan, The Bronx, Brooklyn, Staten Island, Queens
   * - **the_geom**
     - Polygon boundary of the block

And, here are some common SQL aggregation functions you might find useful:

 * avg() - the average (mean) of the values in a set of records
 * sum() - the sum of the values in a set of records
 * count() - the number of records in a set of records

Now the questions:

 * **"What is the population of the City of New York?"**
 
   .. code-block:: sql
   
     SELECT Sum(popn_total) AS population
       FROM nyc_census_blocks;
     
   :: 
   
     8008278 
   
   .. note:: 
   
       What is this ``AS``? You can give a table or a column another name by using an alias.  Aliases can make queries easier to both write and to read. So instead of our outputted column name as ``sum`` we write it **AS** the more readable ``population``. 
       
 * **"What is the population of the Bronx?"**

   .. code-block:: sql
 
     SELECT Sum(popn_total) AS population
       FROM nyc_census_blocks
       WHERE boroname = 'The Bronx';
     
   :: 
   
     1332650 
   
 * **"What is the average number of people living in each housing unit in New York City?"**
 
   .. code-block:: sql

     SELECT Sum(popn_total)/Sum(hous_total) AS popn_per_house
       FROM nyc_census_blocks;

   :: 
   
     2.6503540522400804 
   
 * **"For each borough, what percentage of the population is white?"**

   .. code-block:: sql

     SELECT 
         boroname, 
         100 * Sum(popn_white)/Sum(popn_total) AS white_pct
       FROM nyc_census_blocks
       GROUP BY boroname;

   :: 
   
        boroname    |      white_pct      
     ---------------+---------------------
      Brooklyn      | 41.2005552206888663
      The Bronx     | 29.8655310846808990
      Manhattan     | 54.3594013771837665
      Queens        | 44.0806610271290794
      Staten Island | 77.5968611401579346
 
Function List
-------------

`avg(expression) <http://www.postgresql.org/docs/8.2/static/functions-aggregate.html#FUNCTIONS-AGGREGATE-TABLE>`_: PostgreSQL aggregate function that returns the average value of a numeric column.

`count(expression) <http://www.postgresql.org/docs/8.2/static/functions-aggregate.html#FUNCTIONS-AGGREGATE-TABLE>`_: PostgreSQL aggregate function that returns the number of records in a set of records.

`sum(expression) <http://www.postgresql.org/docs/8.2/static/functions-aggregate.html#FUNCTIONS-AGGREGATE-TABLE>`_: PostgreSQL aggregate function that returns the sum of records in a set of records.