title: Couchbase N1QL query with joined subquery with USE KEYS
body: <p>I need to write a n1ql query which demands another sub-query in select clause. As it is mandatory to use 'USE KEYS' while writing subqueries in n1ql. How to write USE KEYS clause for an inner joined query, below is an example of same case:</p>
  
  <pre><code>select meta(m).id as _ID, meta(m).cas as _CAS, 
     (select c.description 
      from bucketName p join bucketName c on p.categoryId = c.categoryId and p.type='product' and 
      c.type='category' and p.masterId=m.masterId ) as description //--How to use USE KEYS here ?
  from bucketName m where m.type='master' and m.caseId='12345'  
  </code></pre>
  
  <p>My requirment is to fetch some value from another 2 joined tables. however, I simplified above query to make it more understandable. </p>
  
  <ul>
  <li>Please suggest the correct way to implement. </li>
  <li>Also, is writting
  sub-queries in n1ql is better than fetching documents seperatly and
  merging them in coding?</li>
  </ul>