Oops! SQL column swap or SQL column clobber?
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Comments posted to this topic are about the item Oops! SQL column swap or SQL column clobber?
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Thanks for the helpful information...
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Good one Thomas! I was aware that column values will be swapped 🙂
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Actually I'm unable to understand the comments and answers properly. Problem is with the word "CLOBBER".
But I'm aware of UPDATE behaviour and I know there won't be any issues in the query.
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Dineshbabu
Desire to learn new things.. -
hi dinesh,
In that update query, while execute set c1 = c2 the c1 column values will be changed, then in that next statement c2 = c1 how it ill work? so c1 column is called as "clobber". i thing so.... if it is wrong please any one clear out my doubt.......
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Manik
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Manik
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Easy one (thanks Itzik! :-)) to end the week. Thanks!
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Great Question. But the link provided has no information specific to this behavior.
Kingston DhasianHow to post data/code on a forum to get the best help - Jeff Moden
http://www.sqlservercentral.com/articles/Best+Practices/61537/ -
Kingston Dhasian (2/1/2013)
Great Question. But the link provided has no information specific to this behavior.+1
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Dineshbabu
Desire to learn new things.. -
Good Question.
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Gobikannan -
Dineshbabu (1/31/2013)
Actually I'm unable to understand the comments and answers properly. Problem is with the word "CLOBBER".....Same here but after checking this http://en.wikipedia.org/wiki/Clobbering, it kind of eased the question to understand.
Like it says c1 = c2 and c2 = c1, if we do the same in programming language like this
Dim a As Integer
Dim b As Integer
a = 1
b = 2
a = b
b = a
in the 5th line a = 2 and 6th line b is not 1 as a is already 2 so here the original value of a which was 1 is no longer in picture, so the b is written to b itself with its original value.
(i guess, i got this write... 😉 )
ww; Raghu
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The first and the hardest SQL statement I have wrote- "select * from customers" - and I was happy and felt smart. -
Lets drive this a little further:
CREATE TABLE dbo.Test(a INT NOT NULL, B INT NOT NULL)
INSERT dbo.Test (a,b) VALUES(1,2)
INSERT dbo.Test (a,b) VALUES(3,4)
GO
DECLARE @a INT = 100
,@b INT = 100;
UPDATE dbo.Test
SET a = @b-2
, b = @a
,@a = a
,@b = b;
GO
SELECT * FROM dbo.Test
GO
DROP TABLE dbo.Test
GO
This also succeeds. And I find this one really nasty with lots of aspects as to why this actually does work.
Best Regards,
Chris Büttner
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Nice question ..
Dineshbabu (1/31/2013)
Actually I'm unable to understand the comments and answers properly. Problem is with the word "CLOBBER".But I'm aware of UPDATE behaviour and I know there won't be any issues in the query.
Wikipedia explains it as : "clobbering a file or computer memory is overwriting its contents."
Explanations states that
will create a temporary copy of the entire column c1 for later use replacing selective rows of column c2, after column c1 is ‘clobbered’
since the update happens using the temporary copy , so swapping happens perfectly.
Nicely explained by raghvendra..
Clobbering , In other words could mean, Swap the drink by making a cocktail....
<<>>
~ demonfox
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Wondering what I would do next , when I am done with this one :ermm: -
This was removed by the editor as SPAM
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Christian Buettner-167247 (2/1/2013)
Lets drive this a little further:CREATE TABLE dbo.Test(a INT NOT NULL, B INT NOT NULL)
INSERT dbo.Test (a,b) VALUES(1,2)
INSERT dbo.Test (a,b) VALUES(3,4)
GO
DECLARE @a INT = 100
,@b INT = 100;
UPDATE dbo.Test
SET a = @b-2
, b = @a
,@a = a
,@b = b;
GO
SELECT * FROM dbo.Test
GO
DROP TABLE dbo.Test
GO
This also succeeds. And I find this one really nasty with lots of aspects as to why this actually does work.
I understand that you find it nasty. That's because it IS nasty.
Any variable assignment in an UPDATE statement is nasty, and I will celebrate the day this "feature" is removed from the product.
Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
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Hugo Kornelis (2/1/2013)
This also succeeds. And I find this one really nasty with lots of aspects as to why this actually does work.I understand that you find it nasty. That's because it IS nasty.
Any variable assignment in an UPDATE statement is nasty, and I will celebrate the day this "feature" is removed from the product.[/quote]
You're not a fan of the old quirky update then? Much as I dislike the obtuseness of it, I like the performance and admire the ingenuity of it. It does seem to provide results orders of magnitude higher than other methods in specific circumstances - running totals say.
http://www.simple-talk.com/sql/performance/writing-efficient-sql-set-based-speed-phreakery/
I had not quite registered the use of 'clobber' for such a circumstance (generally it seems to be an unexpected overwriting that breaks desired behaviour). It's a great word and I shall be looking for chances to wheel it out though :hehe:
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