Sql queries regarding dates - sql

Question

select sysdate
from dual;
SYSDATE
-------
03-May-17
select sysdate -1
from dual;
SYSDATE
-------
02-May-17
==============
If the run the below sql query
select TO_DATE('01-DEC-15','DD-MON-YYYY') - 1
FROM DUAL;
I am getting the answer
30-Nov-15
=====
Can any body help me understand this behaviour?

Answer 1

The documentation has a section on datetime/interval arithmetic. A number added to or subtracted from a date is treated as a fraction of a day:
For example, SYSDATE + 1 is tomorrow. SYSDATE - 7 is one week ago. SYSDATE + (10/1440) is ten minutes from now
As #mathguy pointed out, all dates in Oracle have a time component, and sysdate has the current server time. You can see what's happening if you display the full value of the date/time. You can do that explicitly with to_char() and a suitable format, but for brevity here I'll alter my session NLS settings:
alter session set nls_date_format = 'SYYYY-MM-DD HH24:MI:SS';
select sysdate, sysdate - 1
from dual;
SYSDATE SYSDATE-1
-------------------- --------------------
2017-05-03 16:17:47 2017-05-02 16:17:47
Both show the same time, but on different days.
You can truncate a date to reduce it's precision; by default trunc(sysdate) zeros the time elements so it becomes midnight today rather than the current time.
Your third example is slightly more interesting because you are using mismatching values and format masks; with a 2-digit year and a 4-digit YYYY mask the year doesn't end up as you might have expected:
select to_date('01-DEC-15','DD-MON-YYYY'), to_date('01-DEC-15','DD-MON-YYYY') - 1
from dual;
TO_DATE('01-DEC-15', TO_DATE('01-DEC-15',
-------------------- --------------------
0015-12-01 00:00:00 0015-11-30 00:00:00
That probably isn't what you wanted. The RRRR mask is more forgiving, as described in the manual:
select to_date('01-DEC-15','DD-MON-RRRR'), to_date('01-DEC-15','DD-MON-RRRR') - 1
from dual;
TO_DATE('01-DEC-15', TO_DATE('01-DEC-15',
-------------------- --------------------
2015-12-01 00:00:00 2015-11-30 00:00:00
but it would be better to provide a full 4-digit year anyway:
select to_date('01-DEC-2015','DD-MON-YYYY'), to_date('01-DEC-2015','DD-MON-YYYY') - 1
from dual;
TO_DATE('01-DEC-2015 TO_DATE('01-DEC-2015
-------------------- --------------------
2015-12-01 00:00:00 2015-11-30 00:00:00
or even more simply, use a date literal:
select date '2015-12-01', date '2015-12-01' - 1
from dual;
DATE'2015-12-01' DATE'2015-12-01'-1
-------------------- --------------------
2015-12-01 00:00:00 2015-11-30 00:00:00
This also avoids issues with the date language; 'APR' won't be recognised in sessions where the language - specifically NLS_DATE_LANGAUGE - isn't English (unless it's coincidentally a recognised abbreviation ion another language, but some months will still fail). It's better to use month numbers. If you must use names or abbreviations, you can specify the language they are in with the optional third argument to to_date(). But literals are still easier if you aren't converting from a string variable.