Need SQL to generate multiple rows from subquery - sql

Question

I have a query that selects a month and year from a complex heirachy of tables. To simplify this question, we can use this query:
select 2 as month, 2018 as year from dual;
I need SQL that will use that query as a subquery to output 3 row: that month as well as with the preceeding 2 months (including years). So the output needed for that specific case would be:
Month Year
2 2018
1 2018
12 2017
I have no idea how to proceed. Ideas anyone?

Answer 1

If you have the month/year already in a DATE column, it would be one less conversion, but if they are separate columns like you have provided, you can use a query like the one below.
Query
WITH sample_data AS (SELECT 2 AS month, 2018 AS year FROM DUAL)
SELECT TO_CHAR (ADD_MONTHS (TO_DATE (month || '-' || year, 'MM-YYYY'), ((LEVEL * -1) + 1)), 'MM')
AS month,
TO_CHAR (ADD_MONTHS (TO_DATE (month || '-' || year, 'MM-YYYY'), ((LEVEL * -1) + 1)), 'YYYY')
AS year
FROM sample_data
CONNECT BY LEVEL <= 3;
Result
MONTH YEAR
________ _______
02 2018
01 2018
12 2017

Answer 2

For Oracle 12+:
select *
from
(select 2 as month, 2018 as year
from dual) complex_query
,lateral(
select
extract(month from add_months(dt, 1-level)) as month,
extract(year from add_months(dt, 1-level)) as year
from (
select
to_date(complex_query.year||'-'||complex_query.month||'-01', 'yyyy-mm-dd') as dt
from dual
)
connect by level<=3
);
For previous versions:
select
complex_query.*
,extract(year from (to_date(year||'-'||month,'yyyy-mm')-delta)) year_2
,extract(month from (to_date(year||'-'||month,'yyyy-mm')-delta)) month_2
from
(select 2 as month, 2018 as year
from dual) complex_query
,(select
NUMTOYMINTERVAL(level-1,'month') delta
from dual
connect by level<=3
) v
Results:
MONTH YEAR YEAR_2 MONTH_2
---------- ---------- ---------- ----------
2 2018 2018 1
2 2018 2017 12
2 2018 2017 11

Answer 3

Would union work?
with primary_month as (
select 2 as month, 2018 as year from dual
)
select (case when month = 1 then 13 else month end) - 1 as month , case when month = 1 then year-1 else year end as year from primary_month
union
select month, year from primary_month
union
select month + 1 as month, year from primary_month
;
This example won't handle boundary cases like if the primary month is January, since there is no month 0, but I'm not sure if that's what you're going for and it depends on the real query and schema (e.g. are you using timestamp columns?)