SQL & PL/SQL :: Analytical Query To Find First And Last Value In Date?

My table has two date columns EFF_DT which is the start date and TERM_DT is the end date. The EFF_DT of the next record should be the next date of the TERM_DT record.

My table looks like this.

Input Table:
-----------
CK_IDPI_IDEFF_DT TERM_DT
Mem1ABC1-Jan-1331-Mar-13
Mem1ABC1-Apr-1331-May-13
Mem1ABC1-Jun-1330-Sep-13
Mem1ABC15-Oct-1331-Dec-13
Mem1ABC1-Jan-1431-Mar-14
Mem1XYZ1-Apr-1430-Jun-14
Mem1XYZ1-Jul-1431-Dec-14

Expected Output:
----------------
CK_IDPI_IDEFF_DT TERM_DT
Mem1ABC1-Jan-1330-Sep-13
Mem1ABC15-Oct-1331-Mar-14
Mem1XYZ1-Apr-1431-Dec-14

In the fourth record, the effective date should be 1-Oct-13 which is the next date to the last TERM_DT 30-Sep-13.As the is the break in the date, the output should show 15-Oct-13 sa the second start date.

Note: Refer to the PI_ID columns, there is a break in the date for the sale PI_ID 'ABC'.

Here I am trying to generate a pseudo column, so that the table with the pseudo column looks like as shown below. and I can use first_value and LAST_value by partitioning on the pseudo column to get the desired output.

1) CNT_VAL is the pseudo column:
-----------------------------
CK_IDPI_IDEFF_DT TERM_DT CNT_VAL
Mem1ABC1-Jan-1331-Mar-131
Mem1ABC1-Apr-1331-May-131
Mem1ABC1-Jun-1330-Sep-131

[code].....

My Query :
----------

I not getting the desired output here as the value in pseudo column is 3.

select CK_ID, PI_ID,EFF_DT,TERM_DT,
(case
when case_CONT - LAG(case_CONT,1) over (ORDER BY EFF_DT) = 0 then to_char(case_CONT)
when case_CONT - LAG(case_CONT,1) over (ORDER BY EFF_DT) <> 0 then to_char(LAG(case_CONT,1) over (ORDER BY EFF_DT) + 1)
else to_char(nvl(case_CONT,0))

[code].....

Scripts:
--------------------
Create table lead_test(
CK_ID varchar2(10),
PI_IDvarchar2(10),
EFF_DTDate,
TERM_DT date);

[code].....