Swapping values of variables

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Swapping values of variables

Thomas Franz

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April 19, 2023 at 12:00 am

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Comments posted to this topic are about the item Swapping values of variables
God is real, unless declared integer.

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webrunner SSC-Dedicated Points: 31862 More actions · Answer 1

Thanks for this very instructive question. I didn't realize that the swap works in UPDATE statements - if that had been the question of the day, I would have gotten it wrong.

-- webrunner

-------------------
A SQL query walks into a bar and sees two tables. He walks up to them and asks, "Can I join you?"
Ref.: http://tkyte.blogspot.com/2009/02/sql-joke.html

Rose Bud Hall of Fame Points: 3147 More actions · Answer 2

Rose Bud

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Points: 3147

April 19, 2023 at 1:41 pm

#4176299

Back to the basics question - love it!

Jeff Moden SSC Guru Points: 1004321 More actions · Answer 3

Great question and even better explanation, Thomas. Nicely done.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Carlo Romagnano SSC-Insane Points: 22668 More actions · Answer 4

I use this feature a lot, see the following example:

declare @t table (i int identity primary key, name sysname)
declare @i int = 1
,@name sysname

insert into @t
select TOP(10) name 
from sys.objects
ORDER BY name

while 1=1
begin
    select @name = name
          ,@i += 1
      from @t
     where i = @i
     if @@ERROR != 0 OR @@ROWCOUNT < 1 
        break
     print @name
end

P.S. In the UPDATE of your explanation, the swap runs as expected because the source (deleted) and the destination (inserted) are different, so, after the assignement the source fields do not change. Instead, the variables are both source and destination.

DECLARE @i INT = 1
      , @j INT = 2

CREATE TABLE #tmp (i INT, j INT)
INSERT INTO #tmp (i, j) VALUES (1, 2)

UPDATE #tmp 
   SET @i = @j
     , @j = @i

SELECT @i AS i, @j AS j