title: Apache Calcite to Find Selected Columns in an SQL String
body: <p>I have a use case where I want to know the columns which have been selected in an SQL string.For instance, if the SQL is like this:<br>
  <code>SELECT name, age*5 as intelligence FROM bla WHERE bla=bla</code><br>
  Then, after parsing the above String, I just want the output to be: <code>name, intelligence</code>.<br>
  Firstly, is it possible through Calcite?<br>
  Any other option is also welcome.  </p>
  
  <p>PS: I want to know this before actually running the query on database.</p>