{ ROWS | RANGE }
{ BETWEEN
  { UNBOUNDED PRECEDING
  | CURRENT ROW
  | value_expr { PRECEDING | FOLLOWING }
  } 
AND
  { UNBOUNDED FOLLOWING
  | CURRENT ROW
  | value_expr { PRECEDING | FOLLOWING }
  }
| { UNBOUNDED PRECEDING
  | CURRENT ROW
  | value_expr PRECEDING
  }
}