Change value of variable in pl/sql - date

Question

I am running a unix *_sh script but with methods that use pl/sql in it.
Method in the script:
method(){
sqlplus -s ${DB_CONNECTION} << EOF
set echo on;
set timing on;
set serveroutput on;
DECLARE
v_monthNow Date:=to_date('2018-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS');
v_charMonthNow Varchar(5):=to_char(v_monthNow,'MM');
v_charMonthBefore Varchar(5) :=to_char(v_monthNow, 'MM')-1;
BEGIN
dbms_output.Put_line('SYSDATE is '|| v_monthNow ||' || MonthNow is '|| v_charMonthNow);
IF v_charMonthBefore = '0'
set v_charMonthBefore = '12'
dbms_output.Put_line('MonthBefore is '|| v_charMonthBefore);
ELSE
dbms_output.Put_line('MonthBefore is '|| v_charMonthBefore);
END;
/
EOF
}
My question is: how do I set value of v_charMonthBefore to what I want?
Actually if sysdate is Jan 2018, i need v_monthNow=1 and v_charMonthBefore=12
if sysdate is Feb 2018, i need v_monthNow=2 and v_charMonthBefore=1
Other way to do this is not required, i just need to fix what i have now as i am not really familiar with pl/sql.
Thank you

Answer 1

Use CASE to detect the correct value.
My CTE is here just to create all 12 months in a year. You need lines 7 - 10.
SQL> with test as
2 (select to_number(to_char(add_months(trunc(sysdate, 'yyyy'), level - 1), 'mm')) mon
3 from dual
4 connect by level <= 12
5 )
6 select
7 mon as v_month_now,
8 case when mon = 1 then 12
9 else mon - 1
10 end as v_month_before
11 from test;
V_MONTH_NOW V_MONTH_BEFORE
----------- --------------
1 12
2 1
3 2
4 3
5 4
6 5
7 6
8 7
9 8
10 9
11 10
12 11
12 rows selected.
SQL>