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

# Only one distinct value in a group

I want to select rows from a table where ALL the values for a column are the same. For example, these two tabl...

es:

```T1            T2
Num Let       Num Val
1   A         1   R
2   A         2   R
3   C         3   S
4   C         3   S
5   D         3   -
6   A         4   S
7   A         4   S
8   A         5   R```

I want a query that will return all Num where T1.Num = T2.Num and ALL values for T2.Val are 'S'. The query should return:

```Num
-----
4```

The reason 3 is not returned is because at least one T2.Val is NOT an 'S'. Thanks if you can answer this.

Seems pretty straightforward. Try this:

```select T2.Num
from T1
inner
join T2
on T1.Num = T2.Num
group
by T2.Num
having count(distinct T2.Val) = 1
and min(T2.Val) = 'S'```

Since each row of T1 can join to more than one row of T2, a GROUP BY on T2.Num is required. The HAVING clause ensures that any T2.num is returned only when there's exactly one distinct T2.Val value in its group, and this value is 'S'. The MIN function is used, but it could also have been MAX, since there would be only one distinct value.

Note that a WHERE clause to restrict T2 rows to those containing 'S' would be wrong, unless accompanied by a NOT EXISTS correlated subquery to check for any other rows with any other value besides 'S' -- but this would be needlessly complex, so I won't show it.

UPDATE: Reader B. Humphreys emailed me with a concern about the solution above:

My understanding was that "Count(FieldName)" returned a count of NON-NULL values for the captioned field while "Count(*)" returned a count of all records (NULL or not).

If that's the case, will this actually work properly or will it return TRUE even though the field "T2.Val" contains NULLs in addition to one other non-NULL value?

An excellent observation. One way around this is to replace any NULLS with an empty string, and if there are only NULLs, then the MIN() condition would not be 'S':

```select
...
having count(distinct coalesce(T2.Val,'')) = 1
and min(T2.Val) = 'S'```

Mr. Humphreys suggested:

```select
...
having count(T2.Val) = count(*)
and min(T2.Val) = 'S'
and max(T2.Val) = 'S'```

This works perfectly, even in Microsoft Access (which does not support COUNT DISTINCT, at least not in the version I'm using). Thanks, B. Humphreys!

This was last published in December 2003

## 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 ...

• ### Enexis' citizen developers empowered by the Mendix RAD platform

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