如何在 oracle sql 中将 Mon Dec 21 00:00:00 EST 2020 转换为 dd/mm/yyyy

Question

How we can convert "Mon Dec 21 00:00:00 EST 2020" to dd/mm/yyyy in oracle SQL? The date is stored in the datatype NVARCHAR2.

Answer 1

Here is how you can accept the input as an nvarchar2 and give the output as a varchar2 (you can cast it as nvarchar2 if you need that for some reason - see how I did that for the input).

In most cases, you would only need the timestamp with time zone for further computations - converting back to a string should be only the last step, when you prepare the results for a human-readable report. If you need the "result" in further computations, just strip away the outer call to to_char .

The third line of code shows the input . I don't have your data, so I created it from scratch - it is an nvarchar2 string representing a timestamp with time zone, in the format you showed us. Notice the specification of language within the call to to_timestamp_tz ; without it, if your default is something other than English, "Mon" and "Dec" will cause an error to be thrown.

select to_char(
          to_timestamp_tz(
              to_nchar('Mon Dec 21 00:00:00 EST 2020'),
                       'Dy Mon dd hh24:mi:ss TZR yyyy',
                       'nls_date_language = english'),
          'dd/mm/yyyy') as result
from   dual
;

RESULT    
----------
21/12/2020

EDIT

In further comments, the OP asked about different input strings, which have 'EDT' as the time zone component.

That is a problem. EST is not ambiguous, but EDT is, since different time zone regions use the EDT daylight saving time marker, and they don't all change to daylight saving time at the same time every year. Unlike EST, EDT must appear with an actual time zone region name in the input; EDT is really not a time zone region, it is only a DST marker for different regions, and it can't be used by itself to identify the correct region.

The OP stated that all inputs are supposed to be in the 'America/New_York' region. If so, then that can be added (concatenated) to the inputs, and the format model can be changed to add the TZR element, as shown below. I only show the part where we convert from string to timestamp with time zone; the rest doesn't need to change.

Note - ts in the result of the query is of data type timestamp with time zone ; it looks as strings (in a specific format) in the output, because the output always shows strings, but in fact the result is the correct data type for further processing, if needed - for example, if one must select only the rows where the timestamp is in 2021, etc. In particular, one can apply to_char to the result, with the `dd/mm/yyyy' format model, to match the OP's original request.

with
  inputs (ts) as (
    select 'Mon Dec 21 00:00:00 EST 2020' from dual union all
    select 'Thu Mar 18 00:00:00 EDT 2021' from dual
  )
select to_timestamp_tz(ts || ' America/New_York',
                       'Dy Mon dd hh24:mi:ss TZD yyyy TZR',
                       'nls_date_language = english') as ts
from   inputs
;

TS                                          
--------------------------------------------
2020-12-21 00:00:00.000 America/New_York EST
2021-03-18 00:00:00.000 America/New_York EDT