TO_DSINTERVAL的用法：
有了这个函数，可以进行时间的计算，包括天，时，分，秒的计算。

oracle reference:
In the SQL format, days is an integer between 0 and 999999999, hours is an integer between 0 and 23, and minutes and seconds are integers between 0 and 59. frac_secs is the fractional part(小数部分) of seconds between .0 and .999999999. One or more blanks separate days from hours. Additional blanks are allowed between format elements.

Examples:
The following example uses the SQL format to select from the hr.employees table the employees who had worked for the company for at least 100 days on November 1, 2002:

SELECT employee_id, last_name FROM employees
   WHERE hire_date + TO_DSINTERVAL('100 00:00:00')
   <= DATE '2002-11-01'
   ORDER BY employee_id;

EMPLOYEE_ID LAST_NAME
----------- ---------------
        102 De Haan
        203 Mavris
        204 Baer
        205 Higgins
        206 Giet