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POISNULL vs CASE Return Type
 

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    <p>I recently ran into an interesting issue with changing <code>CASE</code> statements to <code>ISNULL</code> functions in TSQL. The query I was working with is used to get some user attributes and permissions for a website I work on. Previously the query had a number of <code>CASE</code> statements similar to the following:</p> <p>NOTE: <code>a.column1</code> in the example is of type <code>bit</code> in the table. Also note that the <code>[CanDoSomething]</code> result column is sometimes used as a string in the website.</p> <pre><code>SELECT ... CASE WHEN a.column1 IS NULL THEN 0 ELSE a.column1 END [CanDoSomething] ... FROM a </code></pre> <p>The DBA replaced these <code>CASE</code> statements with the <code>ISNULL</code> function:</p> <pre><code>SELECT ... ISNULL(a.column1, 0) [CanDoSomething] ... FROM a </code></pre> <p>This seems like a fine change, but it caused something unexpected when retrieving the data in C#. With the previous query, the value of the <code>[CanDoSomething]</code> column when accessed from a <code>DataTable</code> in C# was <code>1</code> or <code>0</code>. When we changed to using <code>ISNULL</code>, the value in C# was then changed to <code>true</code> or <code>false</code> which, when treated as a string, are obviously not the same as <code>1</code> or <code>0</code>.</p> <p>The errors this caused have already been addressed. I'm just curious why <code>ISNULL</code> returns a different value than an equivalent <code>CASE</code> statement and I can't seem to find any answers on Google.</p>
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    1. COTry using COALESCE(a.column1, 0) instead of ISNULL
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    2. CO@MuhammedAli For [performance reasons](http://stackoverflow.com/questions/7408893/using-isnull-vs-using-coalesce-for-checking-a-specific-condtion) (which aren't present in this case), the DBA normally chooses `ISNULL` over `COALESCE`.
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