What should be the outcome of TRUNC('02-MAY-14','HH24')?

Question

I found a bizarre snippet which is confusing me so I thought I'll ask the experts.

Let assume a tableA got following columns with data:

"START_TIME":1399075198
"END_TIME":1399075200
"START_DATE":"02-MAY-14"
"END_DATE":"03-MAY-14" 

Now query 1:

 SELECT MIN(start_date) INTO sdate FROM tableA;

query 2:

 SELECT TRUNC(sdate, 'HH24') + INTERVAL '30' MINUTE from dual;

So if start-date = '02-MAY-14', how would that truncate to 'HH24'?

Answer 1

The expression:

TRUNC(sdate, 'HH24')

cuts off everything from a date that is smaller than an hour, i.e. the minutes and seconds. For the specific date:

TRUNC('02-MAY-14','HH24')

it returns the date unchanged. It only makes sense if the Oracle date contains a time component.

Possibly, your SQL tool (SQL Developer, TOAD etc.) is configured to not display the time part of Oracle dates. So the original date might in fact be 02-MAY-14 09:03:25. Then it would return:

02-MAY-14 09:00:00

You mention the columns START_TIME and END_TIME but don't use them in the SQL queries. What are they for?

Answer 2

As start_date does not have a time part in your example, TRUNC is superfluous here. If however it had a timepart, if for example start_time had been added to start_date before, then TRUNC would remove minutes, seconds and microseconds, only keeping the date and hour because of 'HH24' which means "truncate datetime down to full hour".

Answer 3

In Oracle the date datatype inherently store the time as well.

Try executing the below query. It should clear things up a bit:

SELECT TO_CHAR(SYSDATE,'DD-MON-YYYY HH:MI:SS'), TO_CHAR(TRUNC(SYSDATE,'HH24'),'DD-MON-YYYY HH:MI:SS') FROM DUAL;