How would I add an artificial termination date to the termination date column based on two different dates for the same patient id - oracle

Question

I need to figure out a query that will compare two EFFECTIVE dates for a given patient number with different HMOs and determine which is the later date of the two and then populate a TERMINATION date field for only the older of the two effective dates with the last day of the previous month of the newer effective date of the two. This needs to be done across multiple patient, HMO, effective date combinations in a table.
SELECT * FROM tablename
The output is this:
HMO PATIENT EFFECTIVE TERMINATION
16 221135 01-APR-18
18 221135 01-OCT-17
12 251181 01-SEP-16
16 251181 01-MAR-15
12 271126 01-MAR-15
16 271126 01-DEC-16
12 291141 01-DEC-16
16 291141 01-FEB-19
12 391134 09-MAY-13
16 391134 01-APR-18
What I am trying to do via a query or queries is this:
HMO PATIENT EFFECTIVE TERMINATION
16 221235 01-APR-18
18 221235 01-OCT-17 3/31/2018
12 251381 01-SEP-16
16 251381 01-MAR-15 8/31/2016
12 2711126 01-MAR-15 11/30/2016
16 2711126 01-DEC-16
12 292241 01-DEC-16 1/31/2019
16 292241 01-FEB-19
12 391534 09-MAY-13 31-MAR-19
16 391534 01-APR-18
I've tried using a case statement but it is unsurprisingly creating four rows per patient, hmo combo and populating two of the rows with dates and leaving two blank:
SELECT DISTINCT
S.HMO
,S.PATIENT
,S.EFFECTIVE
,CASE WHEN S.EFFECTIVE > E.EFFECTIVE THEN LAST_DAY(ADD_MONTHS(S.EFFECTIVE, -1))
WHEN S.EFFECTIVE < E.EFFECTIVE THEN LAST_DAY(ADD_MONTHS(E.EFFECTIVE, -1))
ELSE NULL END AS TERMINATION
FROM tablename S INNER JOIN tablename E ON S.PATIENT=E.PATIENT
WHERE S.PATIENT =221135
Any ideas or advice would be welcome.

Answer 1

With sample data you posted:
SQL> select * from tablename order by patient, effective;
HMO PATIENT EFFECTIVE TERMINATIO
---------- ---------- ---------- ----------
18 221135 10/01/2017
16 221135 04/01/2018
16 251181 03/01/2015
12 251181 09/01/2016
12 271126 03/01/2015
16 271126 12/01/2016
6 rows selected.
such a MERGE might do:
SQL> merge into tablename a
2 using (select patient, max(effective) max_effective,
3 min(effective) min_effective
4 from tablename
5 group by patient
6 ) x
7 on (a.patient = x.patient)
8 when matched then update set
9 a.termination = x.max_effective - 1
10 where a.effective = x.min_effective;
3 rows merged.
Result is then
SQL> select * from tablename order by patient, effective;
HMO PATIENT EFFECTIVE TERMINATIO
---------- ---------- ---------- ----------
18 221135 10/01/2017 03/31/2018
16 221135 04/01/2018
16 251181 03/01/2015 08/31/2016
12 251181 09/01/2016
12 271126 03/01/2015 11/30/2016
16 271126 12/01/2016
6 rows selected.
SQL>