Question

I have a table that looks like this one :

+------+------+------------------+
| item | val  |    timestamp     |
+------+------+------------------+
|  1   | 3.66 | 16-05-2011 09:17 | 
|  1   | 2.56 | 16-05-2011 09:47 | 
|  2   | 4.23 | 16-05-2011 09:37 | 
|  3   | 6.89 | 16-05-2011 11:26 | 
|  3   | 1.12 | 16-05-2011 12:11 |
|  3   | 4.56 | 16-05-2011 13:23 |
|  4   | 1.10 | 16-05-2011 14:11 |
|  4   | 9.79 | 16-05-2011 14:23 |
|  5   | 1.58 | 16-05-2011 15:27 |
|  5   | 0.80 | 16-05-2011 15:29 |
|  6   | 3.80 | 16-05-2011 15:29 |
+------+------+------------------+

so, the grand total of all item for the day : 16 May 2011 is : 40.09

Now i want to retrieve which items of this list form an amount of 80% of the grand total.
Let me make an example :

• Grand Total : 40.09
• 80% of the Grand Total : 32.07

starting from the item with more percentage weight on the total amount i want to retrieve the grouped list of the item that form the 80% of the grand total :

+------+------+
| item | val  |
+------+------+
|  3   | 12.57|
|  4   | 10.89|
|  1   |  6.22|
+------+------+

As you can see the elements in the result set are the elements grouped by item code and ordered from the element with greater percentage weight on the grand total descending until reaching the 80% threshold.

From the item 2 onward the items are discarded from the result set because they exceed the threshold of 80%, because :

12.57 + 10.89 + 6.22 + 4.23 > 32.07 (80 % of the grand total )

This is not an homework, this is a real context where i am stumbled and i need to achieve the result with a single query …

The query should run unmodified or with few changes on MySQL, SQL Server, PostgreSQL .

Answer 1

You can do this with a single query:

WITH Total_Sum(overallTotal) as (SELECT SUM(val) 
                                 FROM dataTable), 

     Summed_Items(id, total) as (SELECT id, SUM(val)
                                 FROM dataTable
                                 GROUP BY id),

     Ordered_Sums(id, total, ord) as (SELECT id, total, 
                                         ROW_NUMBER() OVER(ORDER BY total DESC)
                                      FROM Summed_Items),

     Percent_List(id, itemTotal, ord, overallTotal) as (
                  SELECT id, total, ord, total
                  FROM Ordered_Sums
                  WHERE ord = 1
                  UNION ALL
                  SELECT b.id, b.total, b.ord, b.total + a.overallTotal
                  FROM Percent_List as a
                  JOIN Ordered_Sums as b
                  ON b.ord = a.ord + 1
                  JOIN Total_Sum as c
                  ON (c.overallTotal * .8) > (a.overallTotal + b.total))

SELECT id, itemTotal
FROM Percent_List

Which will yield the following:

id      itemTotal
3       12.57  
4       10.89  
1       6.22  

Please note that this will not work in mySQL (no CTEs), and will require a more recent version of postgreSQL to work (otherwise OLAP functions are not supported). SQLServer should be able to run the statement as-is (I think – this was written and tested on DB2). Otherwise, you may attempt to translate this into correlated table joins, etc, but it will not be pretty, if it’s even possible (a stored procedure or re-assembly in a higher level language may then be your only option).