Hi,
today’s article is the fourth part of my tutorial on analytical functions. Today I will deal with the differences between RANGE and ROWS windows. We already learned about the ROWS windows in Part I of this tutorial. Today we will take closer look at the RANGE windows and how they differ from ROWS.
I’ve prepared some test data (you can download them in the download section). The data looks like this:
It contains DAYs from 01/01/2017 till 30/04/2017 and PRODUCT_NO from 1 to 3.
Now let’s take a look at the first example. We will create a query to calculate the monthly sums of the turnover. Then we want to compare the previous and the current month. We could do it for example with window functions.
| SELECT MONTH, TURNOVER AS CUR_MONTH, SUM(TURNOVER) OVER (ORDER BY MONTH ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING ) AS PREV_MONTH FROM ( SELECT DISTINCT to_char(DAY, ‚YYYY-MM‘) AS MONTH, SUM(TURNOVER) OVER (PARTITION BY to_char(DAY, ‚YYYY-MM‘) ) AS TURNOVER from tbl_test ) |
What we are doing here is calculating monthly values first. Then we are working with a ROWS window, which means that for each row (that is returned by the sub query) we calculate a separate window which is defined in this case as ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING. That means that we are just accessing the previous row. For that it is important that we first sort the data what is done by ORDER BY MONTH.
ROWS also means that the window definition is based on physical rows. That’s the reason why we need a sub query. First we have to create “physically” monthly rows. Then we can work with them. If we don’t do it we receive strange results as the physical level would still be on day level. So within the window daily data is accessed and shown.
Now if we use the RANGE instead of the ROWS we don’t need the subquery. I.e. we receive the same result with this:
| SELECT DISTINCT to_char(DAY, ‚YYYY-MM‘) AS MONTH, SUM(TURNOVER) OVER (PARTITION BY to_char(DAY, ‚YYYY-MM‘) ) AS CUR_MONTH, SUM(TURNOVER) OVER (ORDER BY to_number(to_char(DAY, ‚YYYYMM‘)) RANGE BETWEEN 1 PRECEDING AND 1 PRECEDING ) AS PREV_MONTH FROM tbl_test |
Test it yourself. The results are the same. There are two differences. We are using RANGE instead of ROWS and we don’t have a sub query. We don’t need the sub query any longer because the RANGE windows are based on logical blocks or logical value changes. When you take a closer look at the ORDER BY clause in the last example you find that we are not using to_char(…..) but to_number(to_char(….)).
We do that as the range windows only works with dates or numbers in the ORDER BY clause. The expression you are using there is important for that logical value change. All rows with the same value in that expression are treated as one. They are summed up (because we are using SUM) first (that’s what we have done manually with the sub query in the first example) and afterwards the window is defined. If we say one block back we are stepping back one logical change. In our case it would mean that we step back one month and not one row.
I know that is a little difficult to understand. ROWS means we are defining our window based on physical row changes. RANGE means we are defining it by logical value change within the ORDER BY expression.
That also leads to another point. If you use ROWS and do your window on physical rows the database doesn’t really care if the previous month isn’t available. If we take April and March isn’t there the February is taken instead. With RANGE it is different. Here a NULL would be shown for March as the previous month of April doesn’t have a value. Which is correct then.
This was just a small introduction to the RANGE windows. If you are interested in further information on that just check out my BLOG. Maybe I will write another article on that with more examples. Or take a look at my new SQL book. I’ve added a free excerpt on this BLOG as well.
You find the example code for this article in the download section.
If you are interested in this or other advanced Cognos/SQL topics you can also attend training on my corporate homepage. At the moment my open courses are only available in German. Inhouse training and online courses is also available in English language. So check out my trainings (German) or my online courses (English)!
