private String getTableAlias(ExprNodeDesc en) {
      Set<String> s = new HashSet<String>();
      Set<ExprNodeDesc> visited = new HashSet<ExprNodeDesc>();
      boolean singleTableAlias = getTableAliasHelper(en, s, visited);

      if (!singleTableAlias || s.size() == 0) {
        return null;
      }
      StringBuilder ans = new StringBuilder();
      for (String st : s) {
        ans.append(st);
      }
      return ans.toString();
    }
If the given expression has just a single table associated with it,
return the table alias associated with it. Else, return null.
@param en
@return The table alias associated with the expression if there is a single table
reference. Else, return null.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/PartitionColumnsSeparator.java