Convert datetime column to utc time - sql

Question

I would like to achieve the following.
I have a datetime column that i calculated with the following functions.
to_char(to_date(f_sta_date, 'YYYYMMDD') + (f_sta_time)/86400), 'YYYY-MM-DD HH24:MI:SS')
to_char(to_date(f_sta_date, 'YYYYMMDD') + (f_sta_time)/86400), 'YYYY-MM-DD HH24:MI:SS')
T1
f_sta_date f__sta_time f_sto_date f_sto_time DT_sta DT_sto
20191001 6689 20191001 7185 2019-10-01 01:54:49 2019-10-01 01:59:45
Desired table
T1
f_sta_date f_sta_time_id f_sto_date f_sto_time_id DT_sta DT_sto DT_sta_UTC DT_sto_UTC
20191001 6689 20191001 7185 2019-10-01 01:54:49 2019-10-01 01:59:45
What function could i add to the functions above to achieve the desired result?

Answer 1

To start with: to_char() returns a string from a date. So if you want a date, don't use it, ie replace this:
to_char(to_date(f_sta_date, 'YYYYMMDD') + (f_sta_time)/86400), 'YYYY-MM-DD HH24:MI:SS')
To:
to_date(f_sta_date, 'YYYYMMDD') + f_sta_time/86400
Then: when it comes to managing timezones, you need to use the timestamp with time zone datatype instead of date. To convert your date in localtime (ie the time zone of your session, that is defined by SESSIONTIMEZONE) to a timestamp and get the corresponding date/time at UTC, you can do:
cast(to_date(f_sta_date, 'YYYYMMDD') + f_sta_time/86400 as timestamp with time zone)
at time zone 'UTC'
Your query:
select
to_date(f_sta_date, 'YYYYMMDD') + f_sta_time/86400 dt_sta,
to_date(f_sto_date, 'YYYYMMDD') + f_sto_time/86400 dt_sto,
cast(to_date(f_sta_date, 'YYYYMMDD') + f_sta_time/86400 as timestamp)
at time zone 'UTC' dt_sta_utc,
cast(to_date(f_sto_date, 'YYYYMMDD') + f_sto_time/86400 as timestamp)
at time zone 'UTC' dt_sto_utc
from t

Answer 2

From the discussion in comments under your post, it appears that your date-time (which you are already able to calculate) should instead be a timestamp with time zone, specifically "at time zone CET". Perhaps for your purposes date with time zone would suffice (in Oracle, "date" always means date-time); alas, Oracle doesn't have such a data type, we must use timestamp with time zone.
Below I show how to calculate that, in the WITH clause. (Note that I use a different approach for computing even the date-time - I convert your "time" component to seconds directly, using the Oracle function specifically designed for that, instead of arithmetic operations. Also, while Oracle usually allows you to use a number - your date component - as if it were a string, in some cases it doesn't. For example, you can do that in TO_DATE, but you can't in TO_TIMESTAMP. In any case, you should never rely on such implicit conversions; always make them explicit in your code.)
I show how I created the table for testing; then, in the query, in the WITH clause I do a preliminary computation (computing your DT_STA and DT_STO, but as timestamps with time zone); then in the main query I show how to convert to a different time zone.
create table t1 (
f_sta_date number,
f_sta_time number,
f_sto_date number,
f_sto_time number
);
insert into t1 values (20191001, 6689, 20191001, 7185);
commit;
with
prep (f_sta_date, f_sta_time, f_sto_date, f_sto_time, dt_sta, dt_sto) as (
select f_sta_date, f_sta_time, f_sto_date, f_sto_time,
from_tz(to_timestamp(to_char(f_sta_date), 'yyyymmdd')
+ numtodsinterval(f_sta_time, 'second'), 'CET'),
from_tz(to_timestamp(to_char(f_sto_date), 'yyyymmdd')
+ numtodsinterval(f_sto_time, 'second'), 'CET')
from t1
)
select f_sta_date, f_sta_time, f_sto_date, f_sto_time, dt_sta, dt_sto,
dt_sta at time zone 'UTC' as dt_sta_utc,
dt_sto at time zone 'UTC' as dt_sto_utc
from prep
;
F_STA_DATE F_STA_TIME F_STO_DATE F_STO_TIME DT_STA DT_STO DT_STA_UTC DT_STO_UTC
---------- ---------- ---------- ---------- ----------------------- ----------------------- ----------------------- -----------------------
20191001 6689 20191001 7185 2019-10-01 01:51:29 CET 2019-10-01 01:59:45 CET 2019-09-30 23:51:29 UTC 2019-09-30 23:59:45 UTC