data: begin of wa,
        carrid type spfli-carrid,
        connid type spfli-connid,
        fldate type sflight-fldate,
        bookid type sbook-bookid,
       end of wa,
       itab like sorted table of  wa
            with unique key carrid connid fldate bookid.

 select p~carrid p~connid f~fldate b~bookid

   from ( ( spfli as p
            inner join sflight as f on p~carrid = f~carrid and
                                       p~connid = f~connid   )
            inner join sbook   as b on b~carrid = f~carrid and
                                       b~connid = f~connid and
                                       b~fldate = f~fldate   )

 into corresponding fields of table itab

 where p~cityfrom = 'FRANKFURT' and
       p~cityto    = 'NEW YORK' and
       f~seatsmax > f~seatsocc.

 loop at itab into wa.
   at new fldate.

     write: / wa-carrid, wa-connid, wa-fldate.
     endat.
     write / wa-bookid.
     endloop.

현재 inner join 관련구문을 공부하고 있는데요..궁금한점 몇가지 질문드립니다. spfli라는 테이블을 가지고  sflight와

sbook을 inner join하는데요.. 아래 루프문에서 at new fldate 라는 구문의 의미를 잘모르겠어요..

글구 data 선언부에서 sorted table에서의 키를 설정할때 unique와 non-unitque로 설정할때의 차이점을 알려주세요..