PL/SQL, CASE statement or if statement - plsql

Question

I am new to PL/SQL
I have a code like this
SELECT f.code,f.date,f.amt, row_number() OVER (PARTITION BY f.code ORDER BY f.date DESC) ranki
FROM advance.alloc f
and shows
CODE DATE AMT ranki
122 12/31/2016 3 1
122 12/31/2015 7 2
122 12/31/2014 3 3
123 6/30/2015 3 1
125 6/30/2015 2 1
125 12/31/2014 8 2
Logic is this
if DATE = 12/__/__ AND ranki = 1 THEN ranki 1, so 122 picks 12/31/2016 3
if DATE = 6/30/__ AND ranki = 1 AND if ranki = 2 exists THEN then pick the second one,so 125 picks 12/31/2014 8
if 6/30__ and ranki is ONLY 1 shows Blank on date LIKE 123
so I would like to show
122 12/31/2016 3
123 __________ 3
125 12/31/2014 8
How can I code like this PL/SQL?
WHEN to_char(af.date,'MM') = 12 AND af.ranki = 1 THEN af.date END
I could code first logic, but I can not figure out how to code the rest of the logic
Thanks

Answer 1

Why in PL/SQL? Or do you mean "in Oracle SQL"? (The solution below uses standard analytic functions, so it is not specific to Oracle.)
Add more information through analytic functions, in addition to ranki. Extract the month from the row with ranki = 1, and also the total count for each code. Then the WHERE clause can follow your logic step by step.
with
f ( code, dt, amount ) as (
select 122, to_date('12/31/2016', 'mm/dd/yyyy'), 3 from dual union all
select 122, to_date('12/31/2015', 'mm/dd/yyyy'), 7 from dual union all
select 122, to_date('12/31/2014', 'mm/dd/yyyy'), 3 from dual union all
select 123, to_date( '6/30/2015', 'mm/dd/yyyy'), 3 from dual union all
select 125, to_date( '6/30/2015', 'mm/dd/yyyy'), 2 from dual union all
select 125, to_date('12/31/2014', 'mm/dd/yyyy'), 8 from dual
)
-- End of simulated data (for testing purposes only, not part of the solution).
-- SQL query begins BELOW THIS LINE.
select code, case when mth = 12 or ranki = 2 then dt end as dt, amount
from ( select code, dt, amount,
first_value(extract (month from dt))
over (partition by code order by dt desc) as mth,
row_number() over (partition by code order by dt desc) as ranki,
count(*) over (partition by code) as cnt
from f
)
where mth = 12 and ranki = 1
or cnt = 1
or mth = 6 and ranki = 2
;
CODE DT AMOUNT
---- ---------- ------
122 12/31/2016 3
123 3
125 12/31/2014 8