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    <p>I would expect absolutely no discernible difference in runtimes between these two queries.</p> <p>However, I would still advocate <em>against</em> SELECT *, and not for performance reasons. There is a long-established myth that SELECT * is less efficient because the engine has to go look up the column names in the metadata, but the truth is that there is still a lookup to <em>validate</em> the column names you've written, and the additional cost of retrieving the names is going to be unnoticeable by humans regardless of the result set size.</p> <p>The reasons I advocate against SELECT * is that:</p> <ul> <li><p>it is unlikely you need <em>all</em> of the columns from the table (or all rows, but that's a different story). If you are pulling back more columns than you need, you are doing unnecessary I/O and perhaps forcing SQL Server to perform a table/clustered index scan when it could have performed a scan on a much skinnier index.</p></li> <li><p>even if you do need all of the columns, using SELECT * can cause hard-to-detect problems in your code later. What if someone inserts a column into the middle of the table? Drops a column? Adds a column? Renames a column? Some of these will be caught immediately but I've demonstrated cases where this can cause all kinds of hard-to-debug problems.</p></li> </ul> <p><a href="http://sqlblog.com/blogs/aaron_bertrand/archive/2009/10/10/bad-habits-to-kick-using-select-omitting-the-column-list.aspx" rel="nofollow">http://sqlblog.com/blogs/aaron_bertrand/archive/2009/10/10/bad-habits-to-kick-using-select-omitting-the-column-list.aspx</a></p> <hr> <p>As for how CTEs work in general, that's a pretty broad question here. I'd start with these articles:</p> <p><a href="http://www.simple-talk.com/sql/t-sql-programming/sql-server-cte-basics/" rel="nofollow">http://www.simple-talk.com/sql/t-sql-programming/sql-server-cte-basics/</a></p> <p><a href="http://msdn.microsoft.com/en-us/magazine/cc163346.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/magazine/cc163346.aspx</a></p> <p><a href="http://msdn.microsoft.com/en-us/library/ms190766%28v=sql.100%29.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/ms190766%28v=sql.100%29.aspx</a></p>
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    1. POShould both of these CTEs perform the same?
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    1. USAaron Bertrand
    1. USAaron Bertrand
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    1. POShould both of these CTEs perform the same?
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    1. COAlso, can you point me to some article that can explain me how CTE works internally?
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      1. USAnkush Gupta
    2. CO@Ankush I've added some links.
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      1. USAaron Bertrand
 

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