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    <p>For variable assignment, <a href="https://stackoverflow.com/a/5758032/643591">Martin Smith answers this question here</a> referencing <a href="http://msdn.microsoft.com/en-us/library/ms187953.aspx" rel="nofollow noreferrer">MSDN</a>:</p> <blockquote> <p>If there are multiple assignment clauses in a single SELECT statement, SQL Server does not guarantee the order of evaluation of the expressions. Note that effects are only visible if there are references among the assignments.</p> </blockquote> <h2>But...</h2> <p>If we're dealing with tables, instead of with variables, it is a different story.</p> <p>In this case, Sql Server uses an <em>All-At-Once</em> operation, as discussed by <a href="http://books.google.com/books?id=v_T82PlI3UwC&amp;pg=PT470&amp;dq=t-sql%20fundamentals%20all-at-once%20update&amp;hl=en&amp;sa=X&amp;ei=CFkdUZfGDOHFigLEqICgCA&amp;ved=0CDYQ6AEwAA" rel="nofollow noreferrer">Itzik Ben-Gan in T-Sql Fundamentals</a>.</p> <p>This concept states that all expressions in the same logical phase are evaluated as if the same point in time... regardless of their left-to-right position.</p> <p>So when dealing with the corresponding <code>UPDATE</code> statement:</p> <pre><code>DECLARE @Test TABLE ( i INT, j INT ) INSERT INTO @Test VALUES (0, 0) UPDATE @Test SET i = i + 10, j = i SELECT i, j FROM @Test </code></pre> <p>We get the following results:</p> <pre><code>i j ----------- ----------- 10 0 </code></pre> <p>And by using the <em>All-At-Once</em> evaluation... in Sql Server you can swap column values in a table without an intermediate variable / column.</p> <p>Most RBDMSs behave this way <a href="https://stackoverflow.com/a/10693062/643591">as far as I know, but MySql is an exception</a>.</p> <hr> <h2>EDIT:</h2> <blockquote> <p>Note that effects are only visible if there are references among the assignments.</p> </blockquote> <p>I understand this to mean that if you have a <code>SELECT</code> statement such as the following:</p> <pre><code>SELECT @A = ColumnA, @B = ColumnB, @C = ColumnC FROM MyTable </code></pre> <p>Then it doesn't matter what order the assignments are performed in... you'll get the same results no matter what. But if you have something like...</p> <pre><code>SELECT @A = ColumnA, @B = @A, @C = ColumnC FROM MyTable </code></pre> <p>There is now a <strong>reference among the assignments</strong> (<code>@B = @A</code>), and the order that <code>@A</code> and <code>@B</code> are assigned now matters.</p>
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    1. POOrder of execution in SQL Server variable assignment using SELECT
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    1. USMichael Fredrickson
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    1. CO+1, good find on the answer and near-duplicate question. I had debated acknowledging the column swap counter example in my question, but left it out for brevity. Now, should I vote to close my own question?
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      1. USTim Lehner
    2. CO@TimLehner I had a heck of a time finding the question containing Martin's answer... and the question you just posted was always at the top of my google searches... I think that at the very least, this question is still useful as a pointer to the other question, and a slightly extended discussion...
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      1. USMichael Fredrickson
 

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