title: how to set a column as unique indexer on Sqlite
body: <p>I have 3 columns (_id, column1, column2) _id column has been set as autoincrement</p>
  
  <p>In database there are some duplicate records, so I want to prevent duplicate records with setting column1 as unique indexer. How do I set a column as unique indexer on sqlite? Or how do I prevent duplicate records?  </p>