oracle - EXTRACT() Hour in 24 Hour format

Question

I have something like below-

EXTRACT(HOUR from CAST(to_char(tran_datetime,'DD-MON-YYYY HH24:MI:SS') AS TIMESTAMP))

tran_datetime is DATE type. This gives error for some rows saying HOUR must be between 1 and 12, so I understand that it cannot handle Hour in a 24 Hour format (or military time). The below works (obviously)-

EXTRACT(HOUR from CAST(to_char(tran_datetime,'DD-MON-YYYY HH:MI:SS') AS TIMESTAMP)) 

or

EXTRACT(HOUR from CAST(tran_datetime AS TIMESTAMP))  --12 Hr format by default

Is there a way to use EXTRACT() to get the HOUR in 24 Hour format i.e. 15 for 3 PM, 13 for 1 PM etc.

Please Note- to_char(tran_datetime,'HH24') is a very obvious option, but I am looking to use EXTRACT() function specifically.

Answer 1

The problem is not with extract, which can certainly handle 'military time'. It looks like you have a default timestamp format which has HH instead of HH24; or at least that's the only way I can see to recreate this:

SQL> select value from nls_session_parameters
  2  where parameter = 'NLS_TIMESTAMP_FORMAT';

VALUE
--------------------------------------------------------------------------------
DD-MON-RR HH24.MI.SSXFF

SQL> select extract(hour from cast(to_char(sysdate, 'DD-MON-YYYY HH24:MI:SS')
  2  as timestamp)) from dual;

EXTRACT(HOURFROMCAST(TO_CHAR(SYSDATE,'DD-MON-YYYYHH24:MI:SS')ASTIMESTAMP))
--------------------------------------------------------------------------
                                                                        15

alter session set nls_timestamp_format = 'DD-MON-YYYY HH:MI:SS';

Session altered.

SQL> select extract(hour from cast(to_char(sysdate, 'DD-MON-YYYY HH24:MI:SS')
  2  as timestamp)) from dual;

select extract(hour from cast(to_char(sysdate, 'DD-MON-YYYY HH24:MI:SS') as timestamp)) from dual
                              *
ERROR at line 1:
ORA-01849: hour must be between 1 and 12

So the simple 'fix' is to set the format to something that does recognise 24-hours:

SQL> alter session set nls_timestamp_format = 'DD-MON-YYYY HH24:MI:SS';

Session altered.

SQL> select extract(hour from cast(to_char(sysdate, 'DD-MON-YYYY HH24:MI:SS')
  2  as timestamp)) from dual;

EXTRACT(HOURFROMCAST(TO_CHAR(SYSDATE,'DD-MON-YYYYHH24:MI:SS')ASTIMESTAMP))
--------------------------------------------------------------------------
                                                                        15

Although you don't need the to_char at all:

SQL> select extract(hour from cast(sysdate as timestamp)) from dual;

EXTRACT(HOURFROMCAST(SYSDATEASTIMESTAMP))
-----------------------------------------
                                       15