Oracle Date Calculation

1) Oracle Date Calculation
1@@@@extract and to_char function in plsql block
@@@Example: use the extract function to handle the DATA INTERVAL data type.
SYS@ocm> !cat tmp.sql
DECLARE
   --If your table have start_date and end_date, you could calculate
   --how many day your work using extract function. It is also functional in
   --SQL statement.
   date1  VARCHAR2(32767);
BEGIN
   date1 := EXTRACT( DAY FROM ( sysdate - to_date('2003-01-04','yyyy-mm-dd') ) DAY TO SECOND )
        ||' days ' ||
            EXTRACT( HOUR FROM ( sysdate - to_date('2003-01-04','yyyy-mm-dd') ) DAY TO SECOND )
        || ' hours '||
            EXTRACT( MINUTE FROM ( sysdate - to_date('2003-01-04','yyyy-mm-dd') ) DAY TO SECOND )
        || ' minutes '||
            EXTRACT( SECOND FROM ( sysdate - to_date('2003-01-04','yyyy-mm-dd') ) DAY TO SECOND )
        || ' seconds ';
   DBMS_OUTPUT.put_line(date1);
END;
/
SYS@ocm> @tmp.sql
3650 days 12 hours 21 minutes 47 seconds
PL/SQL procedure successfully completed.


@@@Use to_char handle the date data type.
HR@ocm> !cat tmp.sql
DECLARE
   cur_var     SYS_REFCURSOR;
   rec         employees%ROWTYPE;
   l_counter   NUMBER := 0;
BEGIN
   OPEN cur_var FOR
      SELECT * FROM employees;
   LOOP
   FETCH cur_var INTO rec;
   EXIT WHEN cur_var%NOTFOUND;
      IF to_char(rec.hire_date,'YYYY')
         BETWEEN 2000 AND 2003
      THEN
         l_counter := l_counter + 1;
      END IF;
   END LOOP;
   DBMS_OUTPUT.put_line('In 2000-2003, company hire '||l_counter||' employees.');
   CLOSE cur_var;
END;
/
HR@ocm> @tmp.sql
In 2000-2003, company hire 11 employees.
PL/SQL procedure successfully completed.



2@@@@ extract day information in sql statement
@@@
@@@<1> use built-in function to_char to extract the date information, both sql and pl/sql
@@@
SYS@ocm> !cat tmp.sql
SELECT sysdate FROM dual
/
SELECT  to_char(sysdate,'YYYY') years
      , to_char(sysdate,'MM')   month
      , to_char(sysdate,'DD')   day
      , to_char(sysdate,'HH24') hour
      , to_char(sysdate,'MI')   minutes
      , to_char(sysdate,'SS')   seconds
FROM dual
/

SYS@ocm> @tmp.sql
SYSDATE
-------------------
2013-01-04 12:47:07


YEAR MO DA HO MI SE
---- -- -- -- -- --
2013 01 04 12 47 07


@@@
@@@<2> use built-in function extract date and date interval information 
@@@    both sql/plsql.
@@@extract function for DATE INTERVAL data type
SYS@ocm> !cat tmp.sql
SELECT 
    extract( YEAR FROM
             (sysdate - to_date('2003-01-04','yyyy-mm-dd') )
             YEAR TO MONTH )  years
  , extract( MONTH FROM
             (sysdate - to_date('2003-01-04','yyyy-mm-dd') )
             YEAR TO MONTH )  months
FROM dual
/
SELECT
    extract( DAY FROM
             (sysdate - to_date('2003-01-04','yyyy-mm-dd') )
             DAY TO SECOND )  days
  , extract( HOUR FROM
             (sysdate - to_date('2003-01-04','yyyy-mm-dd') )
             DAY TO SECOND )  hours
  , extract( MINUTE FROM
             (sysdate - to_date('2003-01-04','yyyy-mm-dd') )
             DAY TO SECOND )  minutes
  , extract( SECOND FROM
             (sysdate - to_date('2003-01-04','yyyy-mm-dd') )
             DAY TO SECOND )  seconds
FROM dual
/

SYS@ocm> @tmp.sql

     YEARS     MONTHS
---------- ----------
    10        0


      DAYS    HOURS     MINUTES    SECONDS
---------- ---------- ---------- ----------
      3653       12          56      6


@@@extract function for DATE data type,
Note: here could not display hour, minute and second.
SYS@ocm> !cat tmp.sql
SELECT 
    extract( YEAR   FROM (sysdate) ) year
  , extract( MONTH  FROM (sysdate) ) month
  , extract( DAY    FROM (sysdate) ) day
FROM dual
/

SYS@ocm> @tmp.sql

      YEAR    MONTH         DAY
---------- ---------- ----------
      2013        1           4