title: SQL Server - Unique index vs Unique constraint - Re. Duplicate values
body: <p>A <strong>unique index</strong> ensures that the values in the index key columns are unique.
  A <strong>unique constraint</strong>  guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. When a unique constraint is created a corresponding unique index is automatically created on the column(s).</p>
  
  <p><strong>Qusetions</strong>: </p>
  
  <ol>
  <li>Can duplicate values be inserted if we have a unique index on a column and no unique constraint? </li>
  <li>What about existing duplicates in any on the column - will it allow to create unique index or unique constraint?</li>
  </ol>