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    <p>To expand on @dasblinkenlight's explanation:</p> <p>The real issue is that, per <a href="http://technet.microsoft.com/en-us/library/ms181765.aspx">the documentation</a>, <code>case</code> expressions</p> <blockquote> <p>return the highest precedence type from the set of types in *result_expressions* and the optional *else_result_expression*. For more information, see <a href="http://technet.microsoft.com/en-us/library/ms190309.aspx">Data Type Precedence (Transact-SQL)</a>.</p> </blockquote> <p>While <code>isnull()</code></p> <blockquote> <p>returns the same type as *check_expression*</p> </blockquote> <p>In your case, that is a <code>bit</code>. Your <code>case</code> expression:</p> <pre><code>CASE WHEN a.column1 IS NULL THEN 0 ELSE a.column1 END </code></pre> <p>has execution paths that returns two different data types, an <code>int</code> (the literal value <code>0</code>) and a <code>bit</code> (the value of the column). If you look at the <a href="http://technet.microsoft.com/en-us/library/ms190309.aspx">data type precedence chart</a> linked above, <code>int</code> is of higher precedence than <code>bit</code> and thus the <code>bit</code> value is cast to an <code>int</code>.</p> <p>That is because the conversion from <code>bit</code> to <code>int</code> is a <em>widening conversion</em>. Sql Server's <code>bit</code> type essentially, a 1-bit integer value and so no data is lost. The conversion from <code>int</code> to <code>bit</code> is a <em>narrowing conversion</em> and the possibility of data loss exists (at least in concept).</p> <p>You wouldn't have encountered this problem, if the DBA had used <a href="http://msdn.microsoft.com/en-us/library/ms190349.aspx"><code>coalesce()</code></a> instead of <code>isnull()</code>, which would have been my personal choice, as <code>coalesce()</code> is defined to return a type similar to that of <code>case</code>. <code>coalesce()</code></p> <blockquote> <p>Returns the data type of <em>expression</em> with the highest data type precedence. If all expressions are nonnullable, the result is typed as nonnullable.</p> </blockquote>
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    1. COThat's what I was looking for. Well done. Thank you!
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