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    <p>A sparse column doesn't use <strong>4x the amount of space</strong> to store a value, it uses a (fixed) <strong>4 extra bytes</strong> per non-null value. (As you've already stated, a NULL takes 0 space.)</p> <ul> <li><p>So a non-null value stored in a <em>bit</em> column would be 1 bit + 4 bytes = 4.125 bytes. But if 99% of these are NULL, it is still a net savings.</p></li> <li><p>A non-null value stored in a <em>GUID (UniqueIdentifier)</em> column is 16 bytes + 4 bytes = 20 bytes. So if only 50% of these are NULL, that's still a net savings.</p></li> </ul> <p>So the "expected savings" depends strongly on what <em>kind</em> of column we're talking about, and your estimate of what ratio will be null vs non-null. Variable width columns (varchars) are probably a little more difficult to predict accurately.</p> <p>This <a href="http://msdn.microsoft.com/en-us/library/cc280604.aspx" rel="noreferrer">Books Online Page</a> has a table showing <em>what percentage</em> of different data types would need to be null for you to end up with a benefit.</p> <p>So <em>when</em> should you use a Sparse Column? When you expect a significant percentage of the rows to have a NULL value. Some examples that come to mind:</p> <ul> <li>A "<strong>Order Return Date</strong>" column in an order table. You would hope that a very small percent of sales would result in returned products.</li> <li>A "<strong>4th Address</strong>" line in an Address table. Most mailing addresses, even if you need a Department name and a "Care Of" probably don't need 4 separate lines.</li> <li>A "<strong>Suffix</strong>" column in a customer table. A fairly low percent of people have a "Jr." or "III" or "Esquire" after their name.</li> </ul>
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    1. COThere was a suggested edit that it is 4 extra bytes per non-null *field*. This is incorrect, I've rolled this back to say *row*. To clarify with an example: if we have a 100-row table with a GUID field, 10 rows of which have values, 90 rows are NULL. A GUID field is normally 16 bytes, so a normal (non-sparse) GUID would be 16*100 = 1600 bytes. If we made this a sparse field, the space used would instead be only (16+4)*10 = 200 bytes, not (16*10)+4 = 164 bytes. The 4-byte penalty applies to every populated row.
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