SQL & PL/SQL :: How To Calculate Period Date

Following is the scenario:

CREATE TABLE EMP_VACATION(
EMP_NO NUMBER(3),
VAC_TYPE NUMBER(1),
START_DATE DATE,
END_DATE DATE,
START_PERIOD_DATE DATE,
END_PERIOD_DATE DATE,
PRIMARY KEY (EMP_NO,VAC_TYPE,START_DATE))
[code]....

--How to calculate START_PERIOD_DATE and END_PERIOD_DATE for every time insert reocrd, like for 4 cases?
--I need to check start_date with last period date for same employee.

And based upon calculate START_PERIOD_DATE and END_PERIOD_DATE,,,,,