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  • Committer: Stewart Smith
  • Date: 2011-01-14 05:18:39 UTC
  • mto: (2086.1.3 build)
  • mto: This revision was merged to the branch mainline in revision 2087.
  • Revision ID: stewart@flamingspork.com-20110114051839-d4zao1vnsln9ocv0
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docs/groupby.rst
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Group By
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========
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The GROUP BY clause is used to extract only those records that fulfill a specified criterion.
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SQL GROUP BY Syntax ::
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        SELECT column_name, aggregate_function(column_name)
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        FROM table_name
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        WHERE column_name operator value
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        GROUP BY column_name
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**GROUP BY Clause Example**
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The "Activities" table:
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+---------+--------------+--------------+-------------+----------+
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|Id       |ActivityDate  |ActivityType  |ActivityCost | userID   |
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+=========+==============+==============+=============+==========+
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| 1       |2011-01-02    | Sport        |45           |131       |
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+---------+--------------+--------------+-------------+----------+
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| 2       |2011-01-02    | Art          |10           |256       |
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+---------+--------------+--------------+-------------+----------+
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| 3       |2011-01-02    | Music        |25           |022       |
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+---------+--------------+--------------+-------------+----------+
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| 4       |2011-01-02    | Food         |125          |022       |
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+---------+--------------+--------------+-------------+----------+
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| 5       |2011-01-03    | Music        |40           |131       |
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+---------+--------------+--------------+-------------+----------+
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| 6       |2011-01-03    | Food         |20           |175       |
 
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Group By
 
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========
 
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The GROUP BY clause is used to extract only those records that fulfill a specified criterion.
 
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SQL GROUP BY Syntax ::
 
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        SELECT column_name, aggregate_function(column_name)
 
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        FROM table_name
 
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        WHERE column_name operator value
 
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        GROUP BY column_name
 
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**GROUP BY Clause Example**
 
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The "Activities" table:
 
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+---------+--------------+--------------+-------------+----------+
 
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|Id       |ActivityDate  |ActivityType  |ActivityCost | userID   |
 
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+=========+==============+==============+=============+==========+
 
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| 1       |2011-01-02    | Sport        |45           |131       |
 
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+---------+--------------+--------------+-------------+----------+
 
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| 2       |2011-01-02    | Art          |10           |256       |
 
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+---------+--------------+--------------+-------------+----------+
 
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| 3       |2011-01-02    | Music        |25           |022       |
 
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+---------+--------------+--------------+-------------+----------+
 
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| 4       |2011-01-02    | Food         |125          |022       |
 
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+---------+--------------+--------------+-------------+----------+
 
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| 5       |2011-01-03    | Music        |40           |131       |
 
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+---------+--------------+--------------+-------------+----------+
 
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| 6       |2011-01-03    | Food         |20           |175       |
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+---------+--------------+--------------+-------------+----------+
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Running the following simple query::
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        SELECT userID
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        FROM activities
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        GROUP BY userID;
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        GROUP BY userID;
 
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Returns:
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+---------+
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| userID  |
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+=========+
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| 131     |
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+---------+
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| 256     |
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+---------+
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| 022     |
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+---------+
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| 175     |
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+---------+
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(This shows that GROUP BY accepts a column_name and consolidates like customer values.)
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However, GROUP BY is much more powerful when used with an aggregate function. Let's say you want to find the total amount spent by each unique User.
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You could use the following SQL statement: ::
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+---------+
 
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| userID  |
 
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+=========+
 
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| 131     |
 
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+---------+
 
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| 256     |
 
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+---------+
 
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| 022     |
 
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+---------+
 
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| 175     |
 
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+---------+
 
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(This shows that GROUP BY accepts a column_name and consolidates like customer values.)
 
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However, GROUP BY is much more powerful when used with an aggregate function. Let's say you want to find the total amount spent by each unique User.
 
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You could use the following SQL statement: ::
 
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        SELECT userID,SUM(ActivityCost) AS "Activity Total"
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        FROM Activities
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        GROUP BY userID;
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        GROUP BY userID;
 
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The result-set will look like this:
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======    ==============
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userID    Activity Total
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======    ==============
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131       85             
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256       10             
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022       150            
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175       20             
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======    ==============
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With the aggregate SUM() function, SQL can calculate how much each unique user has spent on activities over time.
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We can also use the GROUP BY statement on more than one column, like this: ::
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        SELECT userID,ActivityDate,SUM(ActivityCost) 
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        FROM Activities
 
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======    ==============
 
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userID    Activity Total
 
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======    ==============
 
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131       85
 
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256       10
 
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022       150
 
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175       20
 
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======    ==============
 
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With the aggregate SUM() function, SQL can calculate how much each unique user has spent on activities over time.
 
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We can also use the GROUP BY statement on more than one column, like this: ::
 
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        SELECT userID,ActivityDate,SUM(ActivityCost)
 
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        FROM Activities
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        GROUP BY userID,ActivityDate;
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