Oracle IF statement for Fixed Year - sql

Question

I have a weird condition while filtering oracle data:
if there are >=3 month already in the latest year, then only show data from this year and the last year. eg: now is May 2021, show 2020 Jan-Dec data and 2021 Jan-May data
if there are <3 month in the the latest year, then show data from the year before last year to now. eg: now is Jan 2021, show 2019 Jan-Dec data, 2020 Jan-Dec data and 2021 Jan
I have code like
SELECT * FROM DB
CASE
WHEN MDY_TIME BETWEEN
SELECT MAX(MDY_TIME, YEAR) AS MAX_DATE FROM DB)
WHERE MAX(MDY_TIME, MONTH)>='3'
THEN
I' sure I'm not using it right and I don't know if I'm even using the right field and don't know how to complete the conditions. This is my first time using SQL.
Data looks like:

Answer 1

I am assuming that the MDY_TIME column is a DATE data-type (if it is not then you should change it to be a DATE data-type.)
You want:
SELECT *
FROM DB
WHERE MDY_TIME >= CASE
WHEN EXTRACT( MONTH FROM SYSDATE ) < 3
THEN ADD_MONTHS( TRUNC( SYSDATE, 'YY' ), -24 )
ELSE ADD_MONTHS( TRUNC( SYSDATE, 'YY' ), -12 )
END
AND MDY_TIME < ADD_MONTHS( TRUNC( SYSDATE, 'MM' ), 1 )
Then for the sample data:
CREATE TABLE db (
mdy_time DATE,
year_month VARCHAR2(7)
GENERATED ALWAYS AS ( TO_CHAR( mdy_time, 'YYYY_MM' ) ),
year INTEGER
GENERATED ALWAYS AS ( EXTRACT(YEAR FROM mdy_time) ),
month INTEGER
GENERATED ALWAYS AS ( EXTRACT(MONTH FROM mdy_time) )
);
INSERT INTO db ( mdy_time )
SELECT ADD_MONTHS( TRUNC( SYSDATE, 'MM' ), 1 - LEVEL )
FROM DUAL
CONNECT BY LEVEL <= 36;
The output is:
MDY_TIME
YEAR_MONTH
YEAR
MONTH
2021-05-01 00:00:00
2021_05
2021
5
2021-04-01 00:00:00
2021_04
2021
4
2021-03-01 00:00:00
2021_03
2021
3
2021-02-01 00:00:00
2021_02
2021
2
2021-01-01 00:00:00
2021_01
2021
1
2020-12-01 00:00:00
2020_12
2020
12
2020-11-01 00:00:00
2020_11
2020
11
2020-10-01 00:00:00
2020_10
2020
10
2020-09-01 00:00:00
2020_09
2020
9
2020-08-01 00:00:00
2020_08
2020
8
2020-07-01 00:00:00
2020_07
2020
7
2020-06-01 00:00:00
2020_06
2020
6
2020-05-01 00:00:00
2020_05
2020
5
2020-04-01 00:00:00
2020_04
2020
4
2020-03-01 00:00:00
2020_03
2020
3
2020-02-01 00:00:00
2020_02
2020
2
2020-01-01 00:00:00
2020_01
2020
1
db<>fiddle here

Answer 2

I believe you're looking for something like the below
with max_date_sel as (select max(mdy_time) max_dt from DB)
select * from DB,
max_date_sel
where
case when extract(MONTH from max_dt) >= 3
and extract(YEAR from max_dt) - Year < 2 then 1
when extract(MONTH from max_dt) < 3
and extract(YEAR from max_dt) - Year < 3 then 1 end = 1

Answer 3

I would suggest:
where (extract(month from sysdate) < 3 and
mdy_time >= trunc(sysdate, 'YYYY') - interval '1' year
) or
mdy_time >= trunc(sysdate, 'YYYY')
If you want this based on the most recent year in the table, you can use window functions:
from (select db.*, max(mdy_time) over() as max_mdy_time
from db
) db
where (extract(month from max_mdy_time) < 3 and
mdy_time >= trunc(max_mdy_time, 'YYYY') - interval '1' year
) or
mdy_time >= trunc(max_mdy_time, 'YYYY')