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Predict output
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Predict output
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arup chakraborty
arup chakraborty
Posted Monday, October 13, 2008 9:03 PM
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Comments posted to this topic are about the item
Predict output
Post #585186
premindthomas
premindthomas
Posted Monday, October 13, 2008 11:19 PM
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Hi Dudes
The question is really amazing.Nice ideas.
Waiting for amazing questions.
Post #585205
Carlo Romagnano
Carlo Romagnano
Posted Tuesday, October 14, 2008 12:59 AM
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Without the ORDER BY clause, you can not predict the order of rows.
NULL, 1
and
1, NULL
are both possible.
In the future release of sqlserver or in another DBMS, the order may be different.
Post #585230
Robert-378556
Robert-378556
Posted Tuesday, October 14, 2008 1:25 AM
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UNION ALL simply concats the results, UNION removes the duplicates, the best way to do it is to sort the result, so NULLs come first.
Try this:
SELECT 3 UNION SELECT '2' UNION SELECT NULL
Post #585242
arup chakraborty
arup chakraborty
Posted Tuesday, October 14, 2008 1:26 AM
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Hi Carlo,
I have tested it several times in SQL Server 2000 and 2005. Every time, it gives NULL first. I guess that it shows the NULL first because NULL has no values. Then it shows the values.
Post #585243
GilaMonster
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Posted Tuesday, October 14, 2008 1:59 AM
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Null comes first because SQL has to do a sort behind the scenes to ensure uniqueness (required by union). Hence there's an implicit Order by 1 ASC on the statement. When you do an order by, null comes before values.
There's no guarantee that the behaviour will be the same in future versions though.
Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass
Post #585262
Hugo Kornelis
Hugo Kornelis
Posted Tuesday, October 14, 2008 2:14 AM
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arup_kc (10/14/2008)
Hi Carlo,
I have tested it several times in SQL Server 2000 and 2005. Every time, it gives NULL first. I guess that it shows the NULL first because NULL has no values. Then it shows the values.
Hi Arup_kc,
It's great that testing shows consistent results, but that doesn't make this the "only" correct answer. A query without ORDER BY returns a relational set, which (by the very definitions of the relational model) has no inherent order. Limitations of the clients force some order upon that set, but unless an ORDER BY clause is specified, that order is indetermined and prone to change at SQL Server's whim.
Undocumented behaviour, even if confirmed by repeated testing, should never be relied on. I thought that after the "GROUP BY no longer implies sorting" change back in 6.5 and the "TOP 100 PERCENT ... ORDER BY in views no longer orders the view" change in 2005, that should be clear by now!
Hugo Kornelis, SQL Server MVP
Visit my SQL Server blog:
http://sqlblog.com/blogs/hugo_kornelis
Post #585268
Carlo Romagnano
Carlo Romagnano
Posted Tuesday, October 14, 2008 2:26 AM
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GilaMonster (10/14/2008)
Hence there's an implicit Order by 1 ASC on the statement.
Also the DISTINCT clause and the GROUP BY perform an implicit ORDER BY for all cols, but "IMPLICIT" is not the rule nor is written in the BOL.
Post #585275
GilaMonster
GilaMonster
Posted Tuesday, October 14, 2008 2:31 AM
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Carlo Romagnano (10/14/2008)
Also the DISTINCT clause and the GROUP BY perform an implicit ORDER BY for all cols, but "IMPLICIT" is not the rule nor is written in the BOL.
I'm fully aware of that. I was trying to explain why, in this case, on the current versions of SQL, the null always comes first.
Oh, and if you look at the exec plan for this query, SQL's actually not doing a sort at all.
Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass
Post #585277
Anipaul
Anipaul
Posted Tuesday, October 14, 2008 3:34 AM
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Nice and wonderful questions ...:)
Arup great effort
Post #585309
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