Convert two datetimes to timestamps and keep the larger one - oracle19c

Question

I work with Oracle Database 19c and I would like to convert two datetimes (DD/MM/YY HH24:MM:SS) into timestamps and only keep the larger one.
I try several script for the conversion like this one :
SELECT
CODE_ACT_PROD,
LIB,
CAST (DAT_CRE AS TIMESTAMP) AS DATE_CRE_TIMESTAMP,
CAST (DAT_MOD AS TIMESTAMP) AS DATE_MOD_TIMESTAMP
FROM ACTI
WHERE CODE_ACT_PROD
IN (
SELECT CODE_ACT_PROD
FROM ART_COM
WHERE ETAT = 0
)
but the result is not what I want, the datetimes are not convert and I don't know how to keep the larger one.

Answer 1

Use GREATEST:
SELECT CODE_ACT_PROD,
LIB,
CAST (DAT_CRE AS TIMESTAMP) AS DATE_CRE_TIMESTAMP,
CAST (DAT_MOD AS TIMESTAMP) AS DATE_MOD_TIMESTAMP,
CAST(GREATEST(DAT_CRE, DAT_MOD) AS TIMESTAMP) AS greatest_timestamp
FROM ACTI
WHERE CODE_ACT_PROD IN (
SELECT CODE_ACT_PROD
FROM ART_COM
WHERE ETAT = 0
)
Which, for the sample data:
CREATE TABLE acti (
code_act_prod INT,
lib INT,
dat_cre DATE,
dat_mod DATE
);
CREATE TABLE art_com (
code_act_prod INT,
etat INT
);
INSERT INTO acti (code_act_prod, lib, dat_cre, dat_mod)
SELECT 1, 2, SYSDATE - 1, SYSDATE FROM DUAL UNION ALL
SELECT 3, 4, TRUNC(SYSDATE), SYSDATE - 2 FROM DUAL;
INSERT INTO art_com (code_act_prod, etat)
SELECT 1, 0 FROM DUAL UNION ALL
SELECT 3, 0 FROM DUAL;
Outputs:
CODE_ACT_PROD
LIB
DATE_CRE_TIMESTAMP
DATE_MOD_TIMESTAMP
GREATEST_TIMESTAMP
1
2
2021-09-01 08:38:21.000000
2021-09-02 08:38:21.000000
2021-09-02 08:38:21.000000
3
4
2021-09-02 00:00:00.000000
2021-08-31 08:38:21.000000
2021-09-02 00:00:00.000000
db<>fiddle here

Answer 2

Oracle does not have a datetime data type. It has date which has a day and a time to the second. And it has a timestamp which also has a day and a time to the second with optional fractional seconds and time zone. Converting a date to a timestamp would just add fractional seconds which were always 0. Neither date nor timestamp data types have a format. A varchar2 would have a format. If the columns are date data types, your code is syntactically valid. I'm not sure how results you are getting differ from the results you want since you're not showing us your sample data or expected results and you're not telling us what you mean when you say that something isn't converted.
Assuming the two columns are actually of type date, your code appears to be fine and you just want to use the greatest function to get the latest date. See this fiddle
with cte as (
select sysdate dat_cr, sysdate + 1 dat_mod
from dual
)
select cast(dat_cr as timestamp) ts_cr,
cast(dat_mod as timestamp) ts_mod,
cast( greatest( dat_cr, dat_mod ) as timestamp ) ts_greatest
from cte;
TS_CR TS_MOD TS_GREATEST
02-SEP-21 08.25.38.000000 AM 03-SEP-21 08.25.38.000000 AM 03-SEP-21 08.25.38.000000 AM
Note that the conversion of the three timestamps to strings to be displayed to humans is controlled by your session's nls_timestamp_format.
If you want to handle null dates by returning whichever date is not null, you can use a coalesce and a case statement
with cte as (
select sysdate dat_cr, sysdate + 1 dat_mod
from dual
union all
select null, sysdate from dual
union all
select sysdate, null from dual
)
select cast(dat_cr as timestamp) ts_cr,
cast(dat_mod as timestamp) ts_mod,
cast( case when dat_cr is null or dat_mod is null
then coalesce( dat_mod, dat_cr )
else greatest( dat_cr, dat_mod )
end
as timestamp ) ts_greatest
from cte;
See this fiddle