title: What is the difference between Unique Key and Index with IsUnique=Yes?
body: <p>I have a table with a primary key, but I want two other columns to be constrained so the combination of the two is guaranteed always to be unique.</p>
  
  <p>(a dumb example: In a BOOKS table, the IBAN column is the primary key, but the combination of the Title and Author columns should also always be unique.)</p>
  
  <p>In the SQL Server Management Studio it's possible to either create a new Index and set IsUnique to Yes, or I can create a new Unique Key.</p>
  
  <p>What is the difference between the two approaches, and which one suits best for which purposes?</p>