Recently, a colleague brought an interesting query to me – the design for one of their modules required that records in a table be grouped into chunks of a specified integer value and each record within this group be assigned a number starting from 1. Hence, if I had 20 records in a table and the group size was 5 records, I would end up with 4 groups and the records would need to be numbered from 1-5 within each group.
Note here that I do not want a fixed number of groups (I can easily achieve this using NTILE). The groups can reach any number – what I need is a fixed number of elements within a group.
This post documents the first solution that came to my mind. While that particular deliverable is complete by the time I write this post, I look forward to hearing about other solutions/suggestions to achieve this requirement.
Creating the Test Scenario
To create a test scenario, I first create a simple test table and populate it with some test data. To make things interesting, I have inserted a prime number of records.
USE tempdb; GO SET NOCOUNT ON --Safety Check IF OBJECT_ID('#recordDistributionTest','U') IS NOT NULL DROP TABLE #recordDistributionTest; GO --Create the test table CREATE TABLE #recordDistributionTest (rowNum INT IDENTITY(1,1), numVal AS (rowNum * 10) ); GO --Insert 17 records into the test table INSERT INTO #recordDistributionTest DEFAULT VALUES; GO 17
The solution that first came to my mind was to use the modulo operator (%) on the identity/row number value.
For the purposes of this example, I have used a @recordsInAGrop variable which is set to 5.
--Variable to hold the number of records in a group DECLARE @recordsInAGroup INT = 5; --Perform the grouping when fetching the records SELECT rdt.rowNum AS RowNumber, rdt.numVal AS RecordValue, CASE WHEN (rdt.rowNum % @recordsInAGroup) = 0 THEN @recordsInAGroup ELSE (rdt.rowNum % @recordsInAGroup) END AS RowNumberWithinGroup FROM #recordDistributionTest AS rdt; GO
Following is the screenshot of the result obtained from this method.
As can be seen from the result, the first 15 records were grouped into 3 groups of 5 each. The last 2 records don’t belong to any group of the size specified in @recordsInAGroup variable and are therefore easily identifiable for further processing.
Do you have any suggestions on how to achieve this requirement in a much more efficient manner? Do drop in a line with your thoughts/suggestions.
Until we meet next time,