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 tables:

```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!

#### Start the conversation

Send me notifications when other members comment.

## SearchDataManagement

• ### EnterpriseDB looks to grow market for PostgreSQL

Enterprises are increasingly using the open source PostgreSQL database. Read in this Q&A where the growth is coming from and how ...

• ### 3 of the top use cases for graph databases

Graph databases establish many unique relationships between data points. These unusual relationships are beneficial in many use ...

• ### Dell EMC Streaming Data Platform integrates open source technology

Dell combines several open source streaming data technologies, including Apache Kafka, Apache Flink and Pravega, to create a new ...

• ### What Salesforce means for Tableau in the cloud

After the Salesforce acquisition, users have wondered about the future of Tableau in the cloud. Experts weigh in on how the ...

• ### MicroStrategy analytics suite fosters insurance co-op's growth

Using MicroStrategy's BI platform, a Canada-based insurance co-operative has been able to spread analytics to end users ...

• ### Yellowbrick Data, MicroStrategy partnership aims to enhance analytics

The new partnership will enable flexible scalability, support for mixed workloads and multi-cloud support. The goal is to provide...

## SearchSAP

• ### 5 tips for a successful S/4HANA Cloud implementation

Moving to S/4HANA public cloud can help you save on IT maintenance and simplify real-time analysis. Here's a look at what you can...

• ### Take care of data before SAP S/4HANA migration

In this Q&A, Rajesh Rengarethinam of ERP security vendor Appsian discusses why reviewing data security and business processes are...

Business Suite 7 will receive mainstream maintenance until 2027, a decision industry observers say reflects the difficulty of an ...

## SearchSQLServer

• ### SQL Server database design best practices and tips for DBAs

Good database design is a must to meet processing needs in SQL Server systems. In a webinar, consultant Koen Verbeeck offered ...

• ### SQL Server in Azure database choices and what they offer users

SQL Server databases can be moved to the Azure cloud in several different ways. Here's what you'll get from each of the options ...

• ### Using a LEFT OUTER JOIN vs. RIGHT OUTER JOIN in SQL

In this book excerpt, you'll learn LEFT OUTER JOIN vs. RIGHT OUTER JOIN techniques and find various examples for creating SQL ...

## TheServerSide.com

• ### Why the 8 Java primitive data types are not objects

What's the difference between Java primitive types and objects? To start, don't classify primitive types as objects. Let's ...

• ### A brief history of Java: How it forever changed programming

The Java programming language is an important tool in an application developer's quiver. But Java has also undergone many changes...

• ### How to properly perform Java String comparisons

Don't let incorrect Java String object comparisons bog down your code. Follow this tutorial to understand the differences between...

## SearchDataCenter

• ### Learn the main Linux OS components

Linux is one operating choice to run on your infrastructure. Get started with these terms to discover how the OS works and how it...

• ### New Dell EMC PowerEdge servers are built for the rugged edge

New Dell Streaming Media Platform includes two small form factor PowerEdge servers and a Modular Data Center. Dell aims the ...

• ### Initiative aims to improve data center incident reporting

The Data Center Incident Reporting Network hopes to pull back the smoke screen on software and hardware issues to improve ...

## SearchContentManagement

• ### Test yourself on the differences between SharePoint and OneDrive

Businesses use Microsoft SharePoint and OneDrive capabilities for a number of reasons. Test your knowledge on the differences ...

• ### Akeneo rolls out newest version of PIM software

Akeneo PIM version 4.0 features digital asset manager, API-integrated connection modules and AI attribute mapping to better ...

• ### Get to know 4 workflow automation tools

Workflow automation can benefit businesses by making manual processes digital, giving employees more time to work on other tasks....

## SearchHRSoftware

• ### How real-time analytics can benefit HR

For analytics to be truly useful, it must be embedded into user workflows. Learn how HR tools are incorporating such approaches ...

• ### Human vs. AI in recruiting: Why both matter

Recruiters may consider using both the human element and the data brought in by intelligent tools if it can help them make better...

• ### Kronos-Ultimate Software merger creates \$3B firm with product overlap

Kronos Inc. and Ultimate Software are being merged by their private equity owner into a new, yet unnamed, entity. How this will ...

Close