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    copied!<p>For a one-to-many relationship, where the “many” has a small number of known values, relationships can be stored as bitmasks in the parent table as an integer, replacing the need for an additional table.</p> <p>Say we have a table Person and we’d like to know how many Continents a person has visited. We’d start by assigning each Continent an “orthogonal” bit value. In C#, an enum is a good choice for this:</p> <pre><code>[Flags] public enum JobAdvertisingRegion { NorthAmerica = 1, // or 1 &lt;&lt; 0 SouthAmerica = 2, // 1 &lt;&lt; 1 Europe = 4, // 1 &lt;&lt; 2 Asia = 8, // 1 &lt;&lt; 3 Africa = = 16, // 1 &lt;&lt; 4 Australia = 32, // 1 &lt;&lt; 5 Anarctica = 64 // 1 &lt;&lt; 6 } </code></pre> <p>The Persons table could then simply have a int column called Contintents. To indicate that a Person has visited Europe and Asia:</p> <pre><code>UPDATE Persons SET Continents = (4 + 8) WHERE Id = whatever </code></pre> <p>To search for Persons who have visited Antarctica, we use bitwise math:</p> <pre><code>SELECT * FROM Persons WHERE Continents &amp; 64 = 64 </code></pre> <p>To search for Persons who have visited both Africa and Asia:</p> <pre><code>SELECT * FROM Persons WHERE Continents &amp; (16 + 8) = (16 + 8) </code></pre> <p>To search for Persons who have visited either Australia or South America:</p> <pre><code>SELECT * FROM Persons WHERE Continents &amp; (32 + 2) != 0 </code></pre> <p>One downside is that, while integer columns are indexable in SQL, their bit components are not. Some optimizations to get around this, for the above queries:</p> <pre><code>SELECT * FROM Persons WHERE Continents &amp; 64 = 64 AND Continents &gt;= 64 SELECT * FROM Persons WHERE Continents &amp; (16 + 8) = (16 + 8) AND Continents &gt;= (16 + 8) SELECT * FROM Persons WHERE Continents &amp; (32 + 2) != 0 AND Continents &gt;= 2 </code></pre>
 

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