SelectWithUnionQuery (children 1)
 ExpressionList (children 1)
  SelectQuery (children 1)
   ExpressionList (children 2)
    Function greatest (children 1)
     ExpressionList (children 2)
      Literal NULL
      Literal \'a\'
    Function least (children 1)
     ExpressionList (children 2)
      Literal NULL
      Literal \'a\'
