SQL Statement - Working with Dates

The Oracle database stores dates in an internal numeric format, representing the century, year, month, day, hours, minutes, and seconds.
The default display and input format for any date is DD-MON-RR. Valid Oracle dates are between January 1, 4712 B.C., and December 31, 9999 A.D.

RR Date Format

The RR date format is similar to the YY element, but you can use it to specify different centuries. Use the RR date format element instead of YY so that the century of the return value varies according to the specified two-digit year and the last two digits of the current year.

Oracle Date Format
This data is stored internally as follows:
          CENTURY      YEAR      MONTH     DAY    HOUR    MINUTE    SECOND
          19                87          06            17      17         10            43

Centuries and the Year 2000
When a record with a date column is inserted into a table, the century information is picked up from the SYSDATE function. However, when the date column is displayed on the screen, the century component is not displayed (by default).

The DATE data type always stores year information as a four-digit number internally: two digits for the century and two digits for the year. For example, the Oracle database stores the year as 1987 or 2004, and not just as 87 or 04.

Using the SYSDATE Function
SYSDATE is a date function that returns the current database server date and time. You can use SYSDATE just as you would use any other column name. For example, you can display the current date by selecting SYSDATE from a table. It is customary to select SYSDATE from a dummy table called DUAL.

Note: SYSDATE returns the current date and time set for the operating system on which the database resides. Hence, if you are in a place in Australia and connected to a remote database in a location in the United States (US), sysdate function will return the US date and time. In that case, you can use the CURRENT_DATE function that returns the current date in the session time zone.

Arithmetic with Dates
Because the database stores dates as numbers, you can perform calculations using arithmetic operators such as addition and subtraction. You can add and subtract number constants as well as dates.
You can perform the following operations:

        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
SELECT last_name, (SYSDATE-hire_date)/7 AS WEEKS
FROM   employees
WHERE  department_id = 90;

The example displays the last name and the number of weeks employed for all employees in department 90. It subtracts the date on which the employee was hired from the current date (SYSDATE) and divides the result by 7 to calculate the number of weeks that a worker has been employed.

Note: SYSDATE is a SQL function that returns the current date and time. Your results may differ depending on the date and time set for the operating system of your local database when you run the SQL query.
If a more current date is subtracted from an older date, the difference is a negative number.

Date-Manipulation Functions
Date functions operate on Oracle dates. All date functions return a value of the DATE data type except MONTHS_BETWEEN, which returns a numeric value.

   MONTHS_BETWEEN(date1, date2): Finds the number of months between date1 and date2. The result can be positive or negative. If date1 is later than date2, the result is positive; if date1 is earlier than date2, the result is negative. The noninteger part of the result represents a portion of the month.
   ADD_MONTHS(date, n): Adds n number of calendar months to date. The value of n must be an integer and can be negative.
   NEXT_DAY(date, 'char'): Finds the date of the next specified day of the week ('char') following date. The value of char may be a number representing a day or a character string.
   LAST_DAY(date): Finds the date of the last day of the month that contains date

The above list is a subset of the available date functions. ROUND and TRUNC number functions can also be used to manipulate the date values as shown below:

   ROUND(date[,'fmt']): Returns date rounded to the unit that is specified by the format model fmt. If the format model fmt is omitted, date is rounded to the nearest day.
   TRUNC(date[, 'fmt']): Returns date with the time portion of the day truncated to the unit that is specified by the format model fmt. If the format model fmt is omitted, date is truncated to the nearest day.

For example, display the employee number, hire date, number of months employed, six-month review date, first Friday after hire date, and the last day of the hire month for all employees who have been employed for fewer than 100 months.

SELECT employee_id, hire_date,
ADD_MONTHS (hire_date, 6) REVIEW,
NEXT_DAY (hire_date, 'FRIDAY'), LAST_DAY(hire_date)
FROM   employees

Using ROUND and TRUNC Functions with Dates

The ROUND and TRUNC functions can be used for number and date values. When used with dates, these functions round or truncate to the specified format model. Therefore, you can round dates to the nearest year or month. If the format model is month, dates 1-15 result in the first day of the current month. Dates 16-31 result in the first day of the next month. If the format model is year, months 1-6 result in January 1 of the current year. Months 7-12 result in January 1 of the next year.

Compare the hire dates for all employees who started in 1997. Display the employee number, hire date, and starting month using the ROUND and TRUNC functions.

SELECT employee_id, hire_date,
       ROUND(hire_date, 'MONTH'), TRUNC(hire_date, 'MONTH')
FROM   employees
WHERE  hire_date LIKE '%97';