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  • Chapter 03Using SingleRow Functions to Customize Output(03)

    Working with Dates

    • The Oracle database stores dates in an internal numeric format:century,year,month,day,hours,minutes,and seconds.
    • The default date display format is DD-MON-RR.
      • -Enables you to store 21st-century dates in the 20th century by specifying only the last two digits of the year.
      • -Enables you to store 20th-century dates in the 21st century in the same way.
    View Code
    SQL> SELECT last_name,hire_date FROM employees WHERE hire_date < '01-FEB-03';
    
    LAST_NAME                 HIRE_DATE
    ------------------------- ---------
    Mavris                    07-JUN-02
    Baer                      07-JUN-02
    Higgins                   07-JUN-02
    Gietz                     07-JUN-02
    De Haan                   13-JAN-01
    Greenberg                 17-AUG-02
    Faviet                    16-AUG-02
    Raphaely                  07-DEC-02
    
    8 rows selected.

    RR Date Format

    Current Year Specified Date RR Format YY Format
    1995 27-OCT-95 1995 1995
    1995 27-OCT-17 2017 1917
    2001 27-OCT-17 2017 2017
    2001 27-OCT-95 1995 2095
      If the specified two-digit year is:
    0-49 50-99
    If two digits of the current year are: 0-49 The return date is in the current centry The return date is in the century before the current one.
    50-99 The return date is in the century after the current one The return date is in the current century

     

    Using the SYSDATE Function

    SYSDATE is a function that returns:

    • Date
    • Time
    View Code
    SQL> SELECT TO_CHAR(sysdate,'YYYY-MM-DD HH24:MI:SS') CURRENTDATE FROM dual;
    
    CURRENTDATE
    -------------------
    2013-04-12 13:05:31

    Arithmetic With Dates

    • Add or subtract a number to or from a date for a resultant date value.
    • Subtract two dates to find the number of days between those dates.
    • Add hours to a date by dividing the number of hours by 24. 

    Using Arithmetic Operators with Dates

    View Code
    SQL> SELECT last_name,(SYSDATE-hire_date)/7 AS WEEKS FROM employees WHERE department_id = 90;
    
    LAST_NAME                      WEEKS
    ------------------------- ----------
    King                      512.508074
    Kochhar                   394.365217
    De Haan                   638.936645

    Date-Mainpulation Functions

    Function Result
    MONTHS_BETWEEN Number of months between two dates
    ADD_MONTHS Add calendar months to date
    NEXT_DAY Next day of the date specified
    LAST_DAY Last day of the month
    ROUND Round date
    TRUNC Truncate date

    Using Date Functions And Using ROUND and TRUNC Functions with Dates.

    View Code
    SQL> SELECT MONTHS_BETWEEN('01-SEP-95','11-JAN-94') FROM DUAL;
    
    MONTHS_BETWEEN('01-SEP-95','11-JAN-94')
    ---------------------------------------
                                 19.6774194
    
    SQL> SELECT ADD_MONTHS('31-JAN-96',1) FROM DUAL;
    
    ADD_MONTH
    ---------
    29-FEB-96
    
    SQL> SELECT NEXT_DAY('01-SEP-95','FRIDAY') FROM DUAL;
    
    NEXT_DAY(
    ---------
    08-SEP-95
    
    SQL> SELECT LAST_DAY('01-FEB-95') FROM DUAL;
    
    LAST_DAY(
    ---------
    28-FEB-95
    
    SQL>
    View Code
    SQL> SELECT SYSDATE FROM DUAL;
    
    SYSDATE
    ---------
    12-APR-13
    
    SQL> SELECT ROUND(SYSDATE,'MONTH') FROM DUAL;
    
    ROUND(SYS
    ---------
    01-APR-13
    
    SQL> SELECT ROUND(SYSDATE,'YEAR') FROM DUAL;
    
    ROUND(SYS
    ---------
    01-JAN-13
    
    SQL> SELECT TRUNC(SYSDATE,'MONTH') FROM DUAL;
    
    TRUNC(SYS
    ---------
    01-APR-13
    
    SQL> SELECT TRUNC(SYSDATE,'YEAR') FROM DUAL;
    
    TRUNC(SYS
    ---------
    01-JAN-13
    
    SQL>

    Summary

    In this lesson,you should have learned how to:

    • Perform calulations on data using functions.
    • Modify individual data items using funcions.
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  • 原文地址:https://www.cnblogs.com/arcer/p/3016562.html
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