title: SQL: Add a column and classify into categories
body: <p>I have a table which has <code>transaction_id</code> as the primary key and also contains <code>customer_id</code> which is a foreign key. </p>
  
  <p>Now there is a column <code>type</code> which has two values: <code>'Card'</code> and <code>'cash'</code>. </p>
  
  <p>Now some of the customers have used both the methods for payment. I want to add a new column and classify the customers as "Only card" "Only cash" and "Both". </p>
  
  <pre><code>Transaction id      Customer id Type
  1                   100         Card   
  2                   101         Cash 
  3                   102         Card 
  4                   103         Cash
  5                   101         Card
  </code></pre>
  
  <p>So in this table I want a new column 'Type of payment' which classifies customer 101 as Both since he has used both the methods of payment. </p>