How to change this expression to date in ORACLE - oracle12c

Question

How can I change below expression to give me a output in format MMM d, yyyy given the below act_start_date_time is a date column in ORACLE.
select DECODE(MAX(A.NAME), NULL, NULL, max(act_start_date_time))

Answer 1

If act_start_date_time is a column of datatype date then you should not be passing it into the to_date() function. If you do:
to_date(act_start_date_time, 'YYYY/MON/DD HH:MI:SS')
then you are implicitly converting the date to a string using your session's NLS settings, so it's really:
to_date(to_char(act_start_date_time), 'YYYY/MON/DD HH:MI:SS')
which is
to_date(to_char(act_start_date_time, <NLS_DATE_FORMAT>), 'YYYY/MON/DD HH:MI:SS')
which will either error, give you the same date back, or give you a different date, depending on what your current NLS_DATE_FORMAT is and the original value you're looking at. For instance:
alter session set nls_date_format = 'DD-MON-RR';
select to_date(sysdate, 'YYYY/MON/DD HH:MI:SS') from dual;
TO_DATE(S
---------
17-AUG-02
so 2017-08-02 has been translated to [20]02-08-17. Which is not helpful.
To convert a date into a formatted string you need to use the to_char() function instead, e.g. something like:
decode(max(a.name),null,null, to_char(max(rca.act_start_date_time), 'YYYY/MON/DD HH:MI:SS'))
or
to_char(case when max(a.name) is not null then max(rca.act_start_date_time) end, 'YYYY/MON/DD HH:MI:SS')
or to get the format you first asked about use format mask 'FMMon DD, YYYY'. The FM modifier removes leading zeros from the day number. But with all of those, be aware that NLS settings still have an effect; the abbreviated month name will be shown in the session's date language; if you need to make sure it's always shown in a specific language you can supply that as the optional third argument to the function:
select to_char(sysdate, 'FMMon DD, YYYY', 'NLS_DATE_LANGUAGE=ENGLISH') from dual;
TO_CHAR(SYSDATE,'FMMO
---------------------
Aug 2, 2017
select to_char(sysdate, 'FMMon DD, YYYY', 'NLS_DATE_LANGUAGE=FRENCH') from dual;
TO_CHAR(SYSDATE,'FMMONDD,YYYY
-----------------------------
Août 2, 2017
Quick demo with fake data from CTEs and relying on the session language:
with a (id, name) as (select 1, null from dual union all select 2, 'Joe' from dual),
rca (id, act_start_date_time) as (select 1, sysdate from dual union all select 2, sysdate from dual)
select a.id,
max(a.name),
decode(max(a.name), null, null,
to_char(max(rca.act_start_date_time), 'YYYY/MON/DD HH:MI:SS')) as string1,
to_char(case when max(a.name) is not null then max(rca.act_start_date_time) end,
'YYYY/MON/DD HH:MI:SS') as string2,
to_char(case when max(a.name) is not null then max(rca.act_start_date_time) end,
'FMMon DD, YYYY') as string3
from a
join rca on rca.id = a.id
group by a.id;
ID MAX STRING1 STRING2 STRING3
---------- --- -------------------- -------------------- ---------------------
1
2 Joe 2017/AUG/02 11:03:47 2017/AUG/02 11:03:47 Aug 2, 2017
(Without your table structures or sample data I can't comment on whether the aggregation and conversion is appropriate, so adapt as needed for your real scenario...)
The date format models are listed in the documentation.