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