DB2 - use field as a labeled duration for time calculation - db2

Question

Given a table that looks like this:
id task scheduled_date reminder
-- ----------------------- ---------------- --------
1 mail january newsletter 2022-01-01 15 days
I had planned on executing a query to mimic date addition as in
SELECT TASK, SCHEDULED_DATE + 15 DAYS FROM ...
==> 2022-01-16
Unfortunately, using the REMINDER field gives an error:
SELECT TASK, (SCHEDULED_DATE + REMINDER) FROM ...
==>[Code: -182, SQL State: 42816] [SQL0182] A date, time, or timestamp expression not valid.
Is there any way to accomplish using the reminder field as a labeled duration? (I'm using IBMi DB2)

Answer 1

You'll need to convert the string "15 days" into an actual duration.
A date durration is a decimal(8,0) number representing YYYYMMDD
So 15 days would be 00000015
1 year, 00010000
1 year 1 month, one day '00010101`
create table testdur (
datedur decimal(8,0)
);
insert into testdur
values (15), (10000), (10101), (90), (300);
select current_date as curDate
, dateDur
,current_date + dateDur
from testdur;
Results

Answer 2

There is an attempt to implement the interval function available in Db2 for LUW. It supports string expression as a parameter, not just string constant as the built-in one.
The result of this function can participate in whatever allowed date arithmetic.
This works on Db2 for LUW v11.1+ and Db2 for IBM i v7.5+ at least.
create or replace function interval_d (p_interval varchar (100))
returns dec (8)
contains sql
deterministic
no external action
begin atomic
declare v_sign dec (1) default 0;
declare v_pattern varchar (100) default '([+-]? *[0-9]+) *(\w+)';
declare v_y int default 0;
declare v_m int default 0;
declare v_d int default 0;
declare v_occ int default 1;
declare v_num int;
declare v_kind varchar (10);
l1: while 1=1 DO
set v_kind =
lower
(
regexp_substr
(
p_interval
, v_pattern
, 1, v_occ, '', 2
)
);
if v_kind is null then leave l1; end if;
set v_num =
int
(
replace
(
regexp_substr
(
p_interval
, v_pattern
, 1, v_occ, '', 1
)
, ' ', ''
)
);
if sign (v_num) * v_sign < 0 then
signal sqlstate '75001' set message_text = 'Sign of all operands must be the same';
end if;
if v_sign = 0 then set v_sign = sign (v_num); end if;
if v_kind in ('d', 'day', 'days')
then set v_d = v_d + v_num;
elseif v_kind in ('mon', 'mons', 'month', 'months')
then set v_m = v_m + v_num;
elseif v_kind in ('y', 'year', 'years')
then set v_y = v_y + v_num;
else
signal sqlstate '75000' set message_text = 'wrong duration';
end if;
set v_occ = v_occ + 1;
end while l1;
if abs (v_d) > 99 then
set v_m = v_m + v_d / 30, v_d = mod (v_d, 30);
end if;
if abs (v_m) > 99 then
set v_y = v_y + v_m / 12, v_m = mod (v_m, 12);
end if;
return v_y * 10000 + v_m * 100 + v_d;
end
select interval_d (i) as d
from
(
values
('4 years 2 months 3 days')
, ('3 day 4 year 2 month')
, ('-4y -2mon -3d')
) t (i)
D
40203
40203
-40203
fiddle