Q
Problem solve Get help with specific problems with your technologies, process and projects.

# Averages over a span of years -- Part 1

For the following sample relation:

```subject | year | enrolled ----------+---------+------------- subject1 | 1998 | 20 subject1 | 1999 | 23 subject1 | 2000 | 16 subject2 | 1999 | 10 subject2 | 2000 | 21 subject3 | 2000 | 9
```

How would I create a query that calculates the average enrollment for each subject over the years? Thanks!

The answer depends on what is meant by an average "over the years."

Here's a solution involving a straightforward average calculation, using the AVG function:

```select subject , avg(enrolled) as avgamt from subjects group by subject
subject avgamt subject1 19.67 subject2 15.50 subject3 9.00
```

Everything looks okay, right? Each subject has one or more entries in the table, and the average was calculated as the sum per subject divided by the number of rows, right?

But what if the average needs to be calculated over all years in the span of years from 1998 to 2000? How do we deal with the fact that some subjects are missing some years?

What we could do is supply the missing years for each subject. There's more than one way to do this, but here's a simple one. The following query uses the integers table (described in Finding all the dates between two dates, 10 June 2002, and also in Aggregates for date ranges, 4 October 2002). The integers table is joined with the original table in a cross join to generate the desired range of years for each subject:

```select distinct subject , 1998+i as theyear from integers , subjects where i between 0 and 2
subject theyear subject1 1998 subject1 1999 subject1 2000 subject2 1998 subject2 1999 subject2 2000 subject3 1998 subject3 1999 subject3 2000
```

How did we know to use "1998+i" and "i between 0 and 2" in this query? By inspection. Actually, in the general case, inspection would not be used, and instead, additional subqueries would obtain the lowest and highest years from the sample data.

We can now use the results of this cross join as a derived table and join it to the original table. We want to use a left outer join, since we know some rows will not match:

```select allyears.subject , allyears.theyear , enrolled from ( select distinct subject , 1998+i as theyear from integers , subjects where i between 0 and 2 ) as allyears left outer join subjects on allyears.subject = subjects.subject and allyears.theyear = subjects.theyear order by allyears.subject , allyears.theyear
subject theyear enrolled subject1 1998 20 subject1 1999 23 subject1 2000 16 subject2 1998 - subject2 1999 10 subject2 2000 21 subject3 1998 - subject3 1999 - subject3 2000 9
```

Okay, that looks fine. So let's try the averages again:

```select allyears.subject , avg(enrolled) as avgamt from ( select distinct subject , 1998+i as theyear from integers , subjects where i between 0 and 2 ) as allyears left outer join subjects on allyears.subject = subjects.subject and allyears.theyear = subjects.theyear group by allyears.subject
subject avgamt subject1 19.67 subject2 15.50 subject3 9.00
```

Uh oh. These are our original results. How can this be?

The explanation is that aggregate functions exclude NULLs. Please see Part 2 of this answer for more information on working with NULLs and aggregates.

This was last published in November 2002

## Content

Find more PRO+ content and other member only offers, here.

#### Have a question for an expert?

Get answers from a TechTarget expert on whatever's puzzling you.

You will be able to add details on the next page.

#### Start the conversation

Send me notifications when other members comment.

## SearchDataManagement

• ### Apache Hadoop 3.0 goes GA, adds hooks for cloud and GPUs

Is this the post-Hadoop era? Not in the eyes of Hadoop 3.0 backers, who see the latest update to the big data framework ...

• ### Expert: For BI, you must know the data integration process

Understanding the data integration process is central to self-service BI and data architecture design, consultant Rick Sherman ...

• ### Graph technology rivals take Amazon Neptune database in stride

Amazon's Neptune database may change the status quo in the graph technology world. But it could also introduce a wider base of ...

• ### Predictive analytics projects can bolster business decisions

Blind faith in predictive models can result in flawed business decisions. Analytics teams need to manage predictive processes ...

• ### How predictive analytics techniques and processes work

Predictive analytics is no longer confined to data scientists and other highly skilled analysts. But other users need to ...

• ### Hyper engine aims to give enterprise Tableau analytics a boost

The Hyper in-memory data engine added in Tableau 10.5 could make Tableau analytics capabilities more expansive, enabling the ...

## SearchSAP

• ### SAP S/4HANA Cloud and indirect access will dominate 2018

Industry experts say SAP S/4HANA Cloud migrations, Leonardo and Cloud Platform are the technology issues for SAP in 2018; on the ...

When a Dutch energy grid provider needed to develop new business apps on top of SAP ERP, it turned to the Mendix RAD platform to ...

• ### SAP's Timo Elliott on enterprise chatbot AI technology

The SAP global innovation evangelist expects AI to affect businesses in three ways: human-computer interaction, automation of ...

## SearchSQLServer

• ### Meltdown and Spectre fixes eyed for SQL Server performance issues

Microsoft has responded to the Spectre and Meltdown chip vulnerabilities with patches and other fixes. But IT teams need to sort ...

• ### Five SQL Server maintenance steps you should take -- ASAP

Putting off SQL Server administration tasks can lead to database problems. Enact these often-neglected maintenance items to help ...

• ### Microsoft Cosmos DB takes Azure databases to a higher level

Azure Cosmos DB brings a new element to the database lineup of Microsoft's cloud platform, offering multiple data models and a ...

## TheServerSide.com

• ### Spring creator Rod Johnson releases API for implementing DevOps

Find out about Rod Johnson's latest project, which promises to simplify the process of implementing DevOps and streamlining the ...

• ### What Java developers need to know about TypeScript syntax

For Java developers transitioning into JavaScript frameworks, like React and Angular, this TypeScript tutorial on syntax will ...

• ### AWS Cloud9 IDE threatens Microsoft developer base

With its Cloud9 IDE, AWS challenges Microsoft where it matters most -- with the developer community, where Microsoft has ...

## SearchDataCenter

• ### Three requirements for a hybrid cloud computing deployment

As the hybrid cloud computing approach gains steam, organizations will need to pay close attention to cross-cloud connectivity ...

• ### Five debunked myths about SSD issues

Solid-state drives are mature now, and the technology has eclipsed hard disk drives with superior performance, manageability and ...

• ### Future data center trends hinge on the edge, cloud and staffing

Edge computing, colocation, cloud and IT staffing issues lead the way as industry analysts make their predictions for the future ...

## SearchContentManagement

• ### Intelligent information management the next wave for ECM

In a 2018 upgrade, M-Files allows users to search for content in multiple repositories, while also being able to automatically ...

• ### SharePoint integration and implementation best practices

Here are some expert advice and tips, as well common definitions, to help make your SharePoint integration and implementation a ...

• ### SharePoint branding capabilities get a facelift

Since Microsoft Ignite last September, SharePoint Online is getting new branding capabilities that have been on the wish lists of...

## SearchFinancialApplications

• ### Finance IT case study: Reporting secrets of Derek Rose

CEO Sacha Rose says specialist reporting tools have saved the company thousands by avoiding unnecessary mistakes.

• ### WestJet turns to gamification to help its Oracle ERP users soar

WestJet's initial gamification project focuses on expense reporting.

• ### The Transformation of HR is Underway

HR is being transformed while we watch.

Close