Overview

http://infolab.stanford.edu/~ullman/fcdb/oracle/or-time.html#overview

Oracle supports both date and time, albeit differently from the SQL2 standard. Rather than using two separate entities, date and time, Oracle only uses one, DATE. The DATE type is stored in a special internal format that includes not just the month, day, and year, but also
the hour, minute, and second.

The DATE type is used in the same way as other built-in types such as INT. For example, the following SQL statement creates a relation with an attribute of type DATE:

create table x(a int, b date);

DATE Format

When a DATE value is displayed, Oracle must first convert that value from the special internal format to a printable string. The conversion is done by a function TO_CHAR, according to a DATE format. Oracle’s default format for DATE is “DD-MON-YY“. Therefore, when you issue the query

select b from x;

you will see something like:

B
---------
01-APR-98

Whenever a DATE value is displayed, Oracle will call TO_CHAR automatically with the default DATE format. However, you may override the default behavior by calling TO_CHAR explicitly with your own DATEformat. For example,

SELECT TO_CHAR(b, 'YYYY/MM/DD') AS b
FROM x;

returns the result:

B
---------------------------------------------------------------------------
1998/04/01

The general usage of TO_CHAR is:

TO_CHAR(<date>, '<format>')

where the <format> string can be formed from over 40 options. Some of the more popular ones include:
, for example.

You have just learned how to output a DATE value using TO_CHAR. Now what about inputting a DATE value? This is done through a function called TO_DATE, which converts a string to a DATE value, again according to the DATE format. Normally, you do not have to call TO_DATE explicitly: Whenever Oracle expects a DATE value, it will automatically convert your input string using TO_DATE according to the defaultDATE format “DD-MON-YY“. For example, to insert a tuple with a DATE attribute, you can simply type:

insert into x values(99, '31-may-98');

Alternatively, you may use TO_DATE explicitly:

insert into x
values(99, to_date('1998/05/31:12:00:00AM', 'yyyy/mm/dd:hh:mi:ssam'));

The general usage of TO_DATE is:

TO_DATE(<string>, '<format>')

where the <format> string has the same options as in TO_CHAR.

Finally, you can change the default DATE format of Oracle from “DD-MON-YY” to something you like by issuing the following command in sqlplus:

alter session set NLS_DATE_FORMAT='<my_format>';

The change is only valid for the current sqlplus session.

The Current Time

The built-in function SYSDATE returns a DATE value containing the current date and time on your system. For example,

select to_char(sysdate, 'Dy DD-Mon-YYYY HH24:MI:SS') as "Current Time"
from dual;

returns

Current Time
---------------------------------------------------------------------------
Tue 21-Apr-1998 21:18:27

which is the time when I was preparing this document  Two interesting things to note here:

• You can use double quotes to make names case sensitive (by default, SQL is case insensitive), or to force spaces into names. Oracle will treat everything inside the double quotes literally as a single name. In this example, if "Current Time" is not quoted, it would have been interpreted as two case insensitive names CURRENT and TIME, which would actually cause a syntax error.
• DUAL is built-in relation in Oracle which serves as a dummy relation to put in the FROM clause when nothing else is appropriate. For example, try “select 1+2 from dual;“.

Another name for the built-in function SYSDATE is CURRENT_DATE. Be aware of these special names to avoid name conflicts.

Operations on DATE

You can compare DATE values using the standard comparison operators such as =, !=, >, etc.

You can subtract two DATE values, and the result is a FLOAT which is the number of days between the two DATE values. In general, the result may contain a fraction because DATE also has a time component. For obvious reasons, adding, multiplying, and dividing two DATE values are not allowed.

You can add and subtract constants to and from a DATE value, and these numbers will be interpreted as numbers of days. For example, SYSDATE+1 will be tomorrow. You cannot multiply or divide DATE values.

With the help of TO_CHAR, string operations can be used on DATE values as well. For example, to_char(<date>, 'DD-MON-YY') like '%JUN%' evaluates to true if <date> is in June.