Date math

The following SQL is selecting data older than one month ago. I need to change it to select data older than 7 days ago. Does anyone know the TO_CHAR(TO_DATE(SYSDATE etc. syntax to calculate a date of seven days ago?

  SELECT * FROM MR_PURCHASED_ITEMS
  WHERE PI_ORDER_DATE <= ADD_MONTHS(SYSDATE,-1) ;

Any information is appreciated.

Thanks,

Linda Received on Tue Jul 25 2000 - 18:50:56 CDT