title: Using data.table syntax to update column
body: <p>I have a data.table DT with 2 columns: ID, QTY</p>
  
  <pre><code>     ID        QTY
      000001       0
      000002       1
      000002       2
      000004       0
      000005       1
      000006       2
  </code></pre>
  
  <p>I want to update the Qty column for those rows where Qty is greater than 1 and replace the corresponding Qty value with 1.
  This is a trivial task that can be performed by using sapply or simply using which command on the column. 
  But I want to know if this can be achieved using the data.table[i,j,by/keyby,.SD] options. </p>
  
  <p>Expected Output:</p>
  
  <pre><code>     ID        QTY
      000001       0
      000002       1
      000002       1
      000004       0
      000005       1
      000006       1
  </code></pre>