Counting NULLs and sub-sets of data

Quick Tip: Counting NULLs and sub-sets of data

You have been asked to produce a chart which will visualize a few data points.

• Total work orders.
• Total closed work orders (work orders with EndDate populated).
• Total open work orders (work orders with a NULL EndDate).
• Total work orders with excessive scrap material (work orders with ScrappedQty >= 100).

You can easily count each of these result sets …

USE AdventureWorks2012
GO
SELECT COUNT(*) [WorkOrder_Count]
FROM Production.WorkOrder wo

SELECT COUNT(*) [EndDate_Count]
FROM Production.WorkOrder wo
WHERE wo.EndDate IS NOT NULL

SELECT COUNT(*) [NULL_EndDate_Count]
FROM Production.WorkOrder wo
WHERE wo.EndDate IS NULL

SELECT COUNT(*) [ExcessiveScrap_Count]
FROM Production.WorkOrder wo
WHERE wo.ScrappedQty >= 100

… but a single query using a single connection would perform much better when returning data for a chart on something like a web-based dashboard. The trouble is that the COUNT() function will not count NULL values by themselves. Instead, it will count all records in the result-set or it will count all non-NULL records based on a given column.

To get around this limitation, you can use the SUM() function with a CASE statement …

SELECT COUNT(*) [WorkOrder_Count]
,COUNT(wo.EndDate) [EndDate_Count]
,SUM(CASE WHEN wo.EndDate IS NOT NULL THEN 1 ELSE 0 END) [EndDate_Count_AnotherWay]
,SUM(CASE WHEN wo.EndDate IS NULL THEN 1 ELSE 0 END) [NULL_EndDate_Count]
,SUM(CASE WHEN wo.ScrappedQty >= 100 THEN 1 ELSE 0 END) [ExcessiveScrap_Count]
FROM Production.WorkOrder wo 

Now you have all of your data points in a single query, with a single record, which is fast and simple for a service or data visualization tool to consume.

Written by Derik Hammer of SQL Hammer

Derik is a data professional focusing on Microsoft SQL Server. His passion focuses around high-availability, disaster recovery, continuous integration, and automated maintenance. his experience has spanned long-term database administration, consulting, and entrepreneurial ventures.

Derik gives the SQL community credit for plugging the gaps in his knowledge when he was a junior DBA and, now that his skills have matured, started SQLHammer.com as one small way to give back and continue the cycle of shared learning.

Derik is the owner and lead author of SQL Hammer, a Microsoft SQL Server resource.

For more information, visit http://www.sqlhammer.com. Follow Derik on Twitter for SQL tips and chat

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