关于oracle的日期计算

 SELECT TRUNC(sysdate, 'dd'),TRUNC(sysdate, 'mm')   FROM dual
 SELECT to_date( TRUNC(sysdate) - to_char(TRUNC(sysdate), 'D')+ 1)  FROM dual;
 select TRUNC(sysdate) - to_char(TRUNC(sysdate), 'D') from dual;
  select TRUNC(sysdate), to_char(TRUNC(sysdate), 'D')   from dual;