如何从 Oracle 中的日期中减去小时数，因此它也会影响当天

Question

I'm trying to subtract date from Oracle so it even effect the day as well. For example, if the timestamp is 01/June/2015 00 hours and if I subtract 2 hours, I want to be able to go to to 31/May/2014 22 hours.

I tried

to_char(sysdate-(2/11), 'MM-DD-YYYY HH24')

but it only subtracts the hour; it does not touch the day itself.

Answer 1

Others have commented on the (incorrect) use of 2/11 to specify the desired interval.

I personally however prefer writing things like that using ANSI interval literals which makes reading the query much easier:

sysdate - interval '2' hour

It also has the advantage of being portable, many DBMS support this. Plus I don't have to fire up a calculator to find out how many hours the expression means - I'm pretty bad with mental arithmetics ;)

Answer 2

Try this:

SELECT to_char(sysdate - (2 / 24), 'MM-DD-YYYY HH24') FROM DUAL

To test it using a new date instance:

SELECT to_char(TO_DATE('11/06/2015 00:00','dd/mm/yyyy HH24:MI') - (2 / 24), 'MM-DD-YYYY HH24:MI') FROM DUAL

Output is: 06-10-2015 22:00, which is the previous day.

Answer 3

sysdate-(2/11)

A day consists of 24 hours . So, to subtract 2 hours from a day you need to divide it by 24 :

DATE_value - 2/24

Using interval for the same:

DATE_value - interval '2' hour

Answer 4

date - n will subtract n days form given date. In order to subtract hrs you need to convert it into day buy dividing it with 24. In your case it should be to_char(sysdate - (2 + 2/24), 'MM-DD-YYYY HH24') . This will subract 2 days and 2 hrs from sysdate.

Answer 5

you should divide hours by 24 not 11
like this:
select to_char(sysdate - 2/24, 'dd-mon-yyyy HH24') from dual