Oracle UTC时间

Question

I'm trying to read an Oracle TIMESTAMP WITH TIMEZONE from a database in UTC-5 (NY) as UTC.

Oracle is driving me crazy:

SELECT 
from_tz(MAX(TIMESTAMPWITHTIMEZONE),'UTC'),
SYS_EXTRACT_UTC(MAX(TIMESTAMPWITHTIMEZONE)),
SYS_EXTRACT_UTC(systimestamp), 
SYSTIMESTAMP AT TIME ZONE 'UTC'
FROM TABLE

Results:

• SYS_EXTRACT_UTC(systimestamp) gives me: 2013-02-20 14:59:04 , which is probably right.

• SYSTIMESTAMP AT TIME ZONE 'UTC' gives me: 2013-02-20 15:59:04 which is my own local Berlin - whatever

Now I want to have TIMESTAMPWITHTIMEZONE (TIMESTAMP(6)) as UTC

• SYS_EXTRACT_UTC(MAX(TIMESTAMPWITHTIMEZONE)) is 2013-02-20 08:55:01

• from_tz(MAX(TIMESTAMPWITHTIMEZONE),'UTC') is 2013-02-20 10:55:01

Srly. Oracle. I want UTC.

Which one is the right one? Or is there a better way?

Answer 1

The functions are different:

• SYS_EXTRACT_UTC converts a TIMESTAMP WITH TIMEZONE to a TIMESTAMP (with inferred but absent timezone=UTC).
• FROM_TZ converts a TIMESTAMP to a TIMESTAMP WITH TIMEZONE

These functions when applied to a single value will in general return a different result:

SQL> SELECT sys_extract_utc(localtimestamp) ext,
  2         from_tz(localtimestamp, 'UTC')  from_tz
  3    FROM dual;

EXT                   FROM_TZ
--------------------- ------------------------
2013/02/20 15:34:24   2013/02/20 16:34:24 UTC

In the first case the TIMESTAMP is implicitly given the timezone of the server and then transformed into the equivalent timestamp at the UTC timezone. Note that in general you should stay away from implicit conversions.

In the second case there is no computation between timezones: the FROM_TZ function adds the geographical location to a point in time variable.

By the way there is something missing in your example: you can't apply the FROM_TZ function on a variable of type TIMESTAMP WITH TIMEZONE (tested on 9ir2 and 11ir2):

SQL> select from_tz(systimestamp, 'UTC') from dual;

select from_tz(systimestamp, 'UTC') from dual

ORA-00932: inconsistent datatypes: 
   expected TIMESTAMP got TIMESTAMP WITH TIME ZONE

---

Edit following comment:

In your case assuming that your column are of time TIMESTAMP , and knowing that they refer to the NY timezone, you could use the AT TIME ZONE expression to convert to UTC :

SQL> SELECT localtimestamp,
  2         from_tz(localtimestamp, 'America/New_York') AT TIME ZONE 'UTC' utc
  3    FROM dual;

LOCALTIMESTAMP        UTC
--------------------- ------------------------
2013/02/20 17:09:09   2013/02/20 22:09:09 UTC

Answer 2

From Oracle 18c you could use TO_UTC_TIMESTAMP_TZ function:

The TO_UTC_TIMESTAMP_TZ function converts any valid ISO 8601 date represented as a string into a TIMESTAMP WITH TIMEZONE, which can optionally be used as input to the SYS_EXTRACT_UTC function.

SELECT TO_UTC_TIMESTAMP_TZ(col_name)
FROM tab_name;