Question

SQL UNIQUE and DISTINCT

Asked by: benpung

Oracle 8i, HP UX server.  Can someone tell me the difference (if any) between the oracle UNIQUE and DISTINCT keywords?  They seem to work exactly the same.  If I say:

SELECT UNIQUE(COLUMN) FROM TABLE

i get the same results as saying

SELECT DISTINCT(COLUMN) FROM TABLE

Is there some internal difference in how to two of them operate on the tables they are selecting from or something?  thanks.

This Question has been solved and asker verified All Experts Exchange premium technology solutions are available to subscription members.

Subscribe now for full access to Experts Exchange and get

Instant Access to this Solution

  • Plus...
  • 30 Day FREE access, no risk, no obligation
  • Collaborate with the world's top tech experts
  • Unlimited access to our exclusive solution database
  • Never be left without tech help again

Subscribe Now

Asked On
2004-10-14 at 11:43:02ID21168753
Tags

unique

,

sql

,

select

Topic

Oracle Database

Participating Experts
7
Points
50
Comments
12

Trusted by hundreds of thousands everyday for fast, accurate and reliable tech support.

  • "The time we save is the biggest benefit of Experts Exchange to Warner Bros. What could take multiple guys 2 hours or more each to find is accessed in around 15 minutes on Experts Exchange." Mike Kapnisakis, Warner Bros.
  • "Our team likes having a resource that is more secure than just using Google and most experts using this service really know their stuff. It's nice to look here first versus using Google." Dayna Sellner, Lockheed Martin
  • "Anytime that I've been stumped with a problem, 9 out of 10 times Experts Exchange has either the accepted solution or an open discussion of the potential solution to the problem." Kenny Red, eBay Inc.

See what Experts Exchange can do for you.

Got a question?

We've got the answer.

Experts Exchange has been collecting answers to technology questions since 1996…3 million and counting! If you have a question, chances are we already have your answer.

Screenshot of Experts Exchange Knowledgebase

Need individual assistance?

Our experts are ready to help.

If you can't find the exact answer you're looking for, ask our exclusive community of 50,000 experts. You’ll get a personalized answer from a trusted professional.

Screenshot of Experts Exchange Knowledgebase

Want to learn from the best?

Read articles from industry experts.

Thousands of free tech tips, tricks, how-to’s and tutorials are available in our peer reviewed articles section. See for yourself how smart our experts are, no login required.

Screenshot of an Article

Working on a long term project?

Store your work and research.

Save solutions to your questions, answers you’ve discovered through searching plus helpful articles in your personal knowledgebase for easy future access.

Screenshot of Experts Exchange Knowledgebase

Access the answers to your technology questions today.

Subscribe Now

30-day free trial. Register in 60 seconds.

What Makes Experts Exchange Unique?

Members of the expert community talk about why the experience at Experts Exchange is different than what you will find anywhere else.

Trusted by the world's most respected brands.

image of each brand's logo

Faithfully serving IT professionals since 1996.

Experts Exchange Logo

Try it out and discover for yourself.

Subscribe Now

30-day free trial. Register in 60 seconds.

Related Solutions

  1. use of keyword "distinct" in Oracle db
    I have just fixed a bug in a Csharp application that was reported because a particular query was returning too much data. The dev environment I was using was working correctly, but the production environent was not. Basically, the exact same query was behaving differently on...
  2. Distinct
    I want to select several fields from a table using distinct but I only want the first three to use the distinct. Is there a way to do this or would I have to use sub query?
  3. distinct
    Hi, How can we show distinct values from cell A1:A100 and place all the distinct value to B1 ? Thanks.

Free Tech Articles

  1. WARNING: 5 Reasons why you should NEVER fix a computer for free.
    It is in our nature to love the puzzle. We are obsessed. The lot of us. We love puzzles. We love the challenge. We thrive on finding the answer. We hate disarray. It bothers us deep in our soul. W...
  2. SCCM OSD Basic troubleshooting
    SCCM 2007 OSD is a fantastic way to deploy operating systems, however, like most things SCCM issues can sometimes be difficult to resolve due to the sheer volume of logs to sift through and the dispe...
  3. Migrate Small Business Server 2003 to Exchange 2010 and Windows 2008 R2
    This guide is intended to provide step by step instructions on how to migrate from Small Business Server 2003 to Windows 2008 R2 with Exchange 2010. For this migration to work you will need the fo...
  4. Create a Win7 Gadget
    This article shows you how to create a simple "Gadget" -- a sort of mini-application supported by Windows 7 and Vista. Gadgets can be dropped anywhere on the desktop to provide instant information, ...
  5. Outlook continually prompting for username and password
    There have been a lot of questions recently regarding Outlook prompting for a username and password whilst using Exchange 2007. There are a few reasons why this would happen and I will try to cover t...
  6. Backup Exchange 2010 Information Store using Windows Backup
    There seems to be quite a lot of confusion around the ability to backup Exchange 2010 using the built in Windows Backup feature. This stems from the omission of this feature prior to Exchange 2007 s...

Cloud Class Webinars

  1. Avoiding Bugs in Microsoft Access
    Alison Balter takes and in-depth look at avoiding bugs in Access. In this webinar you will learn about using the immediate window to debug your applications, invoking the debugger, using breakpoints to troubleshoot, stepping through code, setting the next statement to execute, ...
  2. Top 10 Best New Features in Visio 2010
    Scott Helmers gives live demonstrations of the top 10 new features in Visio 2010. This webinar will teach you how to create compelling diagrams by adding shapes to the page with a single click, linking the shapes in a diagram to data in Excel (or SQL Server, or SharePoint), ...
  3. IT Consultant Business Secrets Revealed
    Michael Munger, Experts Exchange tech pro and IT consultant, pulls back the curtain on his very successful businesses and answers question on every IT consultant and business owner should know about. He shares secrets on what he did to solve the 5 most common problems in IT, ...
  4. Disaster Recovery and Business Continuity
    Quest CTO, Mike Billon, gives an overview of the steps involved in building a dunamic disaster recovery plan. Through case studies and an examination of software/hardware tooles for monitoring and testing, you'll gain a better understandin of where you are, where you want ...
  5. Organize Your Visio Diagrams with Containers and Lists
    Scott Helmers uses cross functional flowcharts, wireframe diagrams, data graphic legends and seating charts to teach you: how to ustilize all three new structured diagram components in Visio 2010, the best practices for organizeing shapes in previous version of Visio, how to organize ...
  6. How to Us Objects, Properties, Events and Methods in Microsoft Access
    Alison Dalter gives an in-depbth look at objects, properties, events and methods in Microsoft Access. In this webinar you will learn about using the object browser, referring to objects, working with properties and methods, working with object variables, understanding the ...

Join the Community

Give a Little. Get a Lot.

Join the community of experts here and help other tech pros by answering question in your area of expertise. You can earn FREE access to all Experts Exchange's premium features and resources.

Join the Community

Answers

 

by: markgeerPosted on 2004-10-14 at 11:58:43ID: 12311173

I've never tried the "unique" keyword in a select statement, so I don't know.  (I have used it often in a "create unique index..." statement, but I didn't even know it was valid in queries.)

Be careful with the "distinct" keyword.  That can give you the results you expect when you select only one column.  If you try it with multiple columns being selected, the results may surprise you - they may not be what you expect.

Also, the "distinct" keywork in a query always forces a sort operation, but depending on the data and the query, that may not be needed.  If it is not needed, it is just a performance penalty to add it.  Some people (especially those with SQL Server experience) tend to add the word "distinct" to most queries apparently out of habit.  That is not a good habit in Oracle.  Usually in Oracle, a group operator (like: count, sum, min, max, avg, etc.) on one column along with a "group by" clause for the other column(s) is more reliable than "distinct" when multiple columns are being returned by a query.

 

by: seazodiacPosted on 2004-10-14 at 12:04:43ID: 12311235

benpung:

THere is a story behind this:

Long, long time ago, Oracle RDBMS has this UNIQUE keyword even before ANSI SQL standard come into birth.

And no doubt, this UNIQUE is the oracle way of get distinct records.

but then ANSI SQL comes along, and say ok, every RDBMS gonna have to use DISTINCT, because this is the standard,

So Oracle add DISTINCT into its sql bank, but Did NOT decommission the UNIQUE.

there you go, so they are coexisting in ORacle.

you won't see anywhere else...

 

by: pankajtiwaryPosted on 2004-10-14 at 20:25:01ID: 12315786

Hi benpung,

Well, there is no difference between distinct and unique. I have seen people who come from Oracle background use distinct and people from other database who follow the holy ANSI sql use unique, but there are no differences as far as the usage is concerned.

Cheers!

 

by: seazodiacPosted on 2004-10-14 at 20:48:55ID: 12315874

No, please don't mislead.

DISTINCT is ANSI standard,

UNIQUE is Product specific...

 

by: andrewstPosted on 2004-10-15 at 02:51:51ID: 12317406

@markgeer,

I feel sure we have had this debate before, but I can't recall the outcome.  You always say that DISTINCT performs worse than GROUP BY; I don't believe that, and in experiments I can't make it so .  Can you provide an example that proves it?

 

by: markgeerPosted on 2004-10-15 at 07:50:19ID: 12320147

No, I have not claimed that "distinct" performs worse than "group by".  I think their performance is equal.  My problem with "distinct" is that the results are often not what I expect, especially when multiple columns are being selected.  If the query is retrieving one column only (as this question is) then there may be no difference.  I wanted the questioner to be aware though that if he/she tries to select multiple columns with "distinct", the results may be surprising and disappointing.  I find the results of "group by" to always be what I expect them to be.

 

by: andrewstPosted on 2004-10-15 at 07:59:39ID: 12320246

Sorry, I misinterpreted you!

 

by: benpungPosted on 2004-10-18 at 05:44:27ID: 12337732

Thanks everyone for your comments.  They were all helpful.

 

by: schubachPosted on 2005-04-21 at 03:13:23ID: 13832101

Could you give an example of when "distinct" doesn't return the results that you would expect with multiple columns?  I often use "distinct" with long queries selecting multiple columns, because it forces uniqueness and also provides ordering, without re-listing the columns in an ORDER BY clause.

 

by: markgeerPosted on 2005-04-21 at 12:20:03ID: 13837240

No, I don't have any examples of when "distinct" gave me different result sets than I expected, mainly because I probably haven't used "distinct" in a multi-column query for some years.  As long as you expect the "distinct" operator to be applied to every column being selected, and not just to the first column, the results you get should match your expectations.  Just remember though that "distinct" will force a sort operation, and in some queries that may not be needed and may add a significant performance penalty.

 

by: ilaskyPosted on 2008-12-27 at 01:56:03ID: 23247795

Distinct means different. Consider this: If in your database, you had shipping methods. Inside the database you have "UPS", "FEDEX", "UPS". If you used distinct in your sql query as stated above, you would get 2 for ups and 1 for fedex. This operator counts different entries in that column and reports to you how many of each category there are. Unique does the same thing. The only operator that I can think of that would give you a different output would be count. Count would just count the items under the column without regard for differences.

 

by: aspire-engPosted on 2010-04-18 at 05:32:26ID: 31102931

far away from the dfiference between Distinct and unique, i just want to know is there any way to use Distinct and sorting the order according to another collumn which has been picked by default,
You know guys that we cannot use Distinct a way explained below:

SELECT DISTINCT CONTENT_ID FROM CONTENT_FEATURE
WHERE OBSOLETE_FLAG = 'N'

ORDER BY FEATURE_DATE.

and it is not helpful at all to put the feature_date in the select because this will mislead ...

Just a thought and same i have faced when i used Group_by, it brings down the pefromace as well as it doesn't give us the result we want particularly if we want tp pick the top 10 records orederd by date ... ..!

20120131-EE-VQP-002

3 Ways to Join

30-Day Free Trial

The Experts

98% positive feedback on 31,087 answers since March 2000. angeliii is a Microsoft Most Valuable Professional for his work with MS SQL Server & Develoment.

He has also proven his knowledge of Visual Basic Programming, PHP Scripting and Oracle Databases.

The Experts

97% positive feedback on 10,752 answers since July 2000. lrmoore has more than 18 years experience in the networking industry.

The six-time Mircosoft MVPs specialties include firewalls, virtual private networking, and network management.

Testimonials

"...and excellent source for support... Kind of like having your very own IT dept." Electriciansnet

Testimonials

"I was apprehensive at signing up at first. However... it has already made my life as an IT administrator much easier." JaCrews

Testimonials

"WOW! You guys have great, active, and knowledgeable people on here." moore50

Business Clients

Business Clients

In the Press

"If you’ve got a question... Experts Exchange can supply an answer.”

In the Press

"...an invaluable aid for both IT professionals and those who require tech support."

In the Press

"where IT professionals provide quick answers on just about any topic"

Business Account Plans

Loading Advertisement...