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POnested SELECT statements interact in ways that I don't understand
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copied!<p>I thought I understood how I can do a <code>SELECT</code> from the results of another <code>SELECT</code> statement, but there seems to be some sort of blurring of scope that I don't understand. I am using SQL Server 2008R2.</p> <p>It is easiest to explain with an example.</p> <p>Create a table with a single nvarchar column - load the table with a single text value and a couple of numbers:</p> <pre><code>CREATE TABLE #temptable( a nvarchar(30) ); INSERT INTO #temptable( a ) VALUES('apple'); INSERT INTO #temptable( a ) VALUES(1); INSERT INTO #temptable( a ) VALUES(2); select * from #temptable; </code></pre> <p>This will return: <code>apple, 1, 2</code> </p> <p>Use <code>IsNumeric</code> to get only the rows of the table that can be cast to numeric - this will leave the text value <code>apple</code> behind. This works fine.</p> <pre><code> select cast(a as int) as NumA from #temptable where IsNumeric(a) = 1 ; </code></pre> <p>This returns: <code>1, 2</code></p> <p>However, if I use that exact same query as an inner select, and try to do a numeric <code>WHERE</code> clause, it fails saying cannot convert <code>nvarchar</code> value 'apple' to data type <code>int</code>. How has it got the value 'apple' back??</p> <pre><code> select x.NumA from ( select cast(a as int) as NumA from #temptable where IsNumeric(a) = 1 ) x where x.NumA > 1 ; </code></pre> <p>Note that the failing query works just fine without the <code>WHERE</code> clause:</p> <pre><code> select x.NumA from ( select cast(a as int) as NumA from #temptable where IsNumeric(a) = 1 ) x ; </code></pre> <p>I find this very surprising. What am I not getting? TIA</p>

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