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Oracle Database - Dates
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  Date Arithmetic - Difference between Dates in Days, Hours, Minutes and/or Seconds

In oracle, the difference (in days and/or fractions thereof) between two DATEs can be found using subtraction:

SELECT DATE '2016-03-23' - DATE '2015-12-25' AS difference FROM DUAL;

Outputs the number of days between the two dates:

DIFFERENCE
----------
        89

And:

SELECT TO_DATE( '2016-01-02 01:01:12', 'YYYY-MM-DD HH24:MI:SS' )
         - TO_DATE( '2016-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS' )
         AS difference
FROM   DUAL

Outputs the fraction of days between two dates:

DIFFERENCE
----------
    1.0425

The difference in hours, minutes or seconds can be found by multiplying this number by 24, 24*60 or 24*60*60 respectively.

The previous example can be changed to get the days, hours, minutes and seconds between two dates using:

SELECT TRUNC( difference                       ) AS days,
       TRUNC( MOD( difference * 24,       24 ) ) AS hours,
       TRUNC( MOD( difference * 24*60,    60 ) ) AS minutes,
       TRUNC( MOD( difference * 24*60*60, 60 ) ) AS seconds
FROM   (
  SELECT TO_DATE( '2016-01-02 01:01:12', 'YYYY-MM-DD HH24:MI:SS' )
         - TO_DATE( '2016-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS' )
         AS difference
  FROM   DUAL

);

(Note: TRUNC() (opens new window) is used rather than FLOOR() (opens new window) to correctly handle negative differences.)

Outputs:

DAYS HOURS MINUTES SECONDS
---- ----- ------- -------
   1     1       1      12

The previous example can also be solved by converting the numeric difference to an interval (opens new window) using NUMTODSINTERVAL() (opens new window):

SELECT EXTRACT( DAY    FROM difference ) AS days,
       EXTRACT( HOUR   FROM difference ) AS hours,
       EXTRACT( MINUTE FROM difference ) AS minutes,
       EXTRACT( SECOND FROM difference ) AS seconds
FROM   (
  SELECT NUMTODSINTERVAL(
           TO_DATE( '2016-01-02 01:01:12', 'YYYY-MM-DD HH24:MI:SS' )
             - TO_DATE( '2016-01-01 00:00:00', 'YYYY-MM-DD HH24:MI:SS' ),
           'DAY'
         ) AS difference
  FROM   DUAL
);
  Setting the Default Date Format Model

When Oracle implicitly converts from a DATE to a string or vice-versa (or when TO_CHAR() (opens new window) or TO_DATE() (opens new window) are explicitly called without a format model) the NLS_DATE_FORMAT session parameter will be used as the format model in the conversion. If the literal does not match the format model then an exception will be raised.

You can review this parameter using:

SELECT VALUE FROM NLS_SESSION_PARAMETERS WHERE PARAMETER = 'NLS_DATE_FORMAT';

You can set this value within your current session using:

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';

(Note: this does not change the value for any other users.)

If you rely on the NLS_DATE_FORMAT to provide the format mask in TO_DATE() or TO_CHAR() then you should not be surprised when your queries break if this value is ever changed.
  Date Arithmetic - Difference between Dates in Months or Years

The difference in months between two dates can be found using the MONTHS_BETWEEN( date1, date2 ) (opens new window):

SELECT MONTHS_BETWEEN( DATE '2016-03-10', DATE '2015-03-10' ) AS difference FROM DUAL;

Outputs:

DIFFERENCE
----------
        12

If the difference includes part months then it will return the fraction of the month based on there being 31 days in each month:

SELECT MONTHS_BETWEEN( DATE '2015-02-15', DATE '2015-01-01' ) AS difference FROM DUAL;

Outputs:

DIFFERENCE
----------
 1.4516129

Due to MONTHS_BETWEEN assuming 31 days per month when there can be fewer days per month then this can result in different values for differences spanning the boundaries between months.

Example:

SELECT MONTHS_BETWEEN( DATE'2016-02-01', DATE'2016-02-01' - INTERVAL '1' DAY ) AS "JAN-FEB",
       MONTHS_BETWEEN( DATE'2016-03-01', DATE'2016-03-01' - INTERVAL '1' DAY ) AS "FEB-MAR",
       MONTHS_BETWEEN( DATE'2016-04-01', DATE'2016-04-01' - INTERVAL '1' DAY ) AS "MAR-APR",
       MONTHS_BETWEEN( DATE'2016-05-01', DATE'2016-05-01' - INTERVAL '1' DAY ) AS "APR-MAY"
FROM   DUAL;

Output:

JAN-FEB FEB-MAR MAR-APR APR-MAY
------- ------- ------- -------
0.03226 0.09677 0.03226 0.06452

The difference in years can be found by dividing the month difference by 12.
  Generating Dates with No Time Component

All DATEs have a time component; however, it is customary to store dates which do not need to include time information with the hours/minutes/seconds set to zero (i.e. midnight).

Use an ANSI DATE literal (opens new window) (using ISO 8601 Date format (opens new window)):

SELECT DATE '2000-01-01' FROM DUAL;

Convert it from a string literal using TO_DATE() (opens new window):

SELECT TO_DATE( '2001-01-01', 'YYYY-MM-DD' ) FROM DUAL;

(More information on the date format models (opens new window) can be found in the Oracle documentation.)

or:

SELECT TO_DATE(
         'January 1, 2000, 00:00 A.M.',
         'Month dd, YYYY, HH12:MI A.M.',
         'NLS_DATE_LANGUAGE = American'
       )
FROM   DUAL;

(If you are converting language specific terms such as month names then it is good practice to include the 3rd nlsparam parameter to the TO_DATE() function and specify the language to be expected.)
  Generating Dates with a Time Component

Convert it from a string literal using TO_DATE() (opens new window):

SELECT TO_DATE( '2000-01-01 12:00:00', 'YYYY-MM-DD HH24:MI:SS' ) FROM DUAL;

Or use a TIMESTAMP literal (opens new window):

CREATE TABLE date_table(
  date_value DATE
);

INSERT INTO date_table ( date_value ) VALUES ( TIMESTAMP '2000-01-01 12:00:00' );

Oracle will implicitly cast a TIMESTAMP to a DATE when storing it in a DATE column of a table; however you can explicitly CAST() (opens new window) the value to a DATE:

SELECT CAST( TIMESTAMP '2000-01-01 12:00:00' AS DATE ) FROM DUAL;
  The Format of a Date

In Oracle a DATE data type does not have a format; when Oracle sends a DATE to the client program (SQL/Plus, SQL/Developer, Toad, Java, Python, etc) it will send 7- or 8- bytes which represent the date.

A DATE which is not stored in a table (i.e. generated by SYSDATE and having "type 13" when using the DUMP() command) has 8-bytes and has the structure (the numbers on the right are the internal representation of 2012-11-26 16:41:09):

BYTE VALUE                           EXAMPLE
---- ------------------------------- --------------------------------------
1    Year modulo 256                 220
2    Year multiples of 256           7   (7 * 256 + 220 = 2012)
3    Month                           11
4    Day                             26
5    Hours                           16
6    Minutes                         41
7    Seconds                         9
8    Unused                          0

A DATE which is stored in a table ("type 12" when using the DUMP() command) has 7-bytes and has the structure (the numbers on the right are the internal representation of 2012-11-26 16:41:09):

BYTE VALUE                           EXAMPLE
---- ------------------------------- --------------------------------------
1    ( Year multiples of 100 ) + 100 120
2    ( Year modulo 100 ) + 100       112 ((120-100)*100 + (112-100) = 2012)
3    Month                           11
4    Day                             26
5    Hours + 1                       17
6    Minutes + 1                     42
7    Seconds + 1                     10

If you want the date to have a specific format then you will need to convert it to something that has a format (i.e. a string). The SQL client may implicitly do this or you can explicitly convert the value to a string (opens new window) using TO_CHAR( date, format_model, nls_params ) (opens new window).
  Converting Dates to a String

Use TO_CHAR( date [, format_model [, nls_params]] ) (opens new window):

(Note: if a format model (opens new window) is not provided then the NLS_DATE_FORMAT session parameter will be used as the default format model (opens new window); this can be different for every session so should not be relied on. It is good practice to always specify the format model.)

CREATE TABLE table_name (
  date_value DATE
);

INSERT INTO table_name ( date_value ) VALUES ( DATE '2000-01-01' );
INSERT INTO table_name ( date_value ) VALUES ( TIMESTAMP '2016-07-21 08:00:00' );
INSERT INTO table_name ( date_value ) VALUES ( SYSDATE );

Then:

SELECT TO_CHAR( date_value, 'YYYY-MM-DD' ) AS formatted_date FROM table_name;

Outputs:

FORMATTED_DATE
--------------
2000-01-01
2016-07-21
2016-07-21

And:

SELECT TO_CHAR(
         date_value,
         'FMMonth d yyyy, hh12:mi:ss AM',
         'NLS_DATE_LANGUAGE = French'
       ) AS formatted_date
FROM   table_name;

Outputs:

FORMATTED_DATE
-----------------------------
Janvier   01 2000, 12:00:00 AM
Juillet   21 2016, 08:00:00 AM
Juillet   21 2016, 19:08:31 PM
  Changing How SQL/Plus or SQL Developer Display Dates

When SQL/Plus or SQL Developer display dates they will perform an implicit conversion to a string using the default date format model (see the Setting the Default Date Format Model (opens new window) example).

You can change how a date is displayed by changing the NLS_DATE_FORMAT parameter.
  Extract the Year, Month, Day, Hour, Minute or Second Components of a Date

The year, month or day components of a DATE data type can be found using the EXTRACT( [ YEAR | MONTH | DAY ] FROM datevalue ) (opens new window)

SELECT EXTRACT (YEAR  FROM DATE '2016-07-25') AS YEAR,
       EXTRACT (MONTH FROM DATE '2016-07-25') AS MONTH,
       EXTRACT (DAY   FROM DATE '2016-07-25') AS DAY
FROM DUAL;

Outputs:

YEAR MONTH DAY
---- ----- ---
2016     7  25

The time (hour, minute or second) components can be found by either:

For example:

SELECT EXTRACT( HOUR   FROM CAST( datetime AS TIMESTAMP ) ) AS Hours,
       EXTRACT( MINUTE FROM CAST( datetime AS TIMESTAMP ) ) AS Minutes,
       EXTRACT( SECOND FROM CAST( datetime AS TIMESTAMP ) ) AS Seconds
FROM   (
  SELECT TO_DATE( '2016-01-01 09:42:01', 'YYYY-MM-DD HH24:MI:SS' ) AS datetime FROM DUAL
);

Outputs:

HOURS MINUTES SECONDS
----- ------- -------
    9      42       1
  Time Zones and Daylight Savings Time

The DATE data type does not handle time zones or changes in daylight savings time.

Either:

A DATE can be stored as Coordinated Universal Time (UTC) and converted to the current session time zone like this:

SELECT FROM_TZ(
         CAST(
           TO_DATE( '2016-01-01 12:00:00', 'YYYY-MM-DD HH24:MI:SS' )
           AS TIMESTAMP
         ),
         'UTC'
       )
       AT LOCAL AS time
FROM   DUAL;

If you run ALTER SESSION SET TIME_ZONE = '+01:00'; then the output is:

TIME
------------------------------------
2016-01-01 13:00:00.000000000 +01:00

and ALTER SESSION SET TIME_ZONE = 'PST'; then the output is:

TIME
------------------------------------
2016-01-01 04:00:00.000000000 PST
  Leap Seconds

Oracle does not handle leap seconds (opens new window). See My Oracle Support note 2019397.2 and 730795.1 for more details.
  Getting the Day of the Week

You can use TO_CHAR( date_value, 'D' ) (opens new window) to get the day-of-week.

However, this is dependent on the NLS_TERRITORY session parameter:

ALTER SESSION SET NLS_TERRITORY = 'AMERICA';        -- First day of week is Sunday
SELECT TO_CHAR( DATE '1970-01-01', 'D' ) FROM DUAL;

Outputs 5

ALTER SESSION SET NLS_TERRITORY = 'UNITED KINGDOM'; -- First day of week is Monday
SELECT TO_CHAR( DATE '1970-01-01', 'D' ) FROM DUAL;

Outputs 4

To do this independent of the NLS settings, you can truncate the date to midnight of the current day (to remove any fractions of days) and subtract the date truncated to the start of the current iso-week (which always starts on Monday):

SELECT TRUNC( date_value ) - TRUNC( date_value, 'IW' ) + 1 FROM DUAL



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