I have created a new method isValidStatement()  where all validations on the SQL statement is done. Currently, the javadoc for it is 

/**
     * The method validates the statement passed to the query plan. List of conditions are
     * <ol>
     *   <li>Is a SELECT statement</li>
     *   <li>doesn't contain ORDER BY expression</li>
     *   <li>doesn't contain LIMIT</li>
     *   <li>doesn't contain GROUP BY expression</li>
     *   <li>doesn't contain DISTINCT</li>
     *   <li>doesn't contain AGGREGATE functions</li>
     * </ol>  

In the final write up document, I will have this specified explicitly.