title: Unique constraint based on weeks in a quarter
body: <p>I am trying to figure out how to apply a unique constraint based on weeks in a quarter.
  I have a table that includes <code>Quarter</code> (1Q2014, 2Q2014, ...) as well as an <code>Effective Date</code> and an <code>Expiration date</code>.</p>
  
  <p>I need to allow multiple records for the same <code>Qtr</code> as long as <code>Effective</code> and <code>Expiration Dates</code> do not overlap.</p>
  
  <p><em>For example, these records would be legitimate:</em></p>
  
  <pre><code>Qtr         EffDte          ExpDte
  --------------------------------------
  1Q2014      2014-01-06      2014-02-03
  
  1Q2014      2014-02-10      2014-02-24
  
  1Q2014      2014-03-03      2014-03-17
  </code></pre>
  
  <p><em>While these records would not be legitimate:</em></p>
  
  <pre><code>Qtr         EffDte          ExpDte
  --------------------------------------
  1Q2014      2014-01-06      2014-03-24
  
  1Q2014      2014-02-10      2014-03-10
  
  1Q2014      2014-03-03      2014-03-17
  </code></pre>
  
  <p>I am not sure if I need to alter my table design to achieve this goal.</p>
  
  <p>Does anyone have any suggestions?</p>