Oracle 11g PL / SQL中的TO_DATE函数是否具有上限/下限

Question

I am new to Oracle development and am still discovering a lot of it's capabilities and the differences between PL/SQL and SQL Server's T-SQL.

So my 'project' is to create a function that would generate a random date from either the future or the past. The user should be able to specify the upper/lower bounds in which the date should be created.

I may be too late but in the interest of breivity I will skip the details of why the function does what it does and how. What is important and pertinent to the actual questions is the following behavoir.

BEGIN
  SYS.DBMS_OUTPUT.PUT_LINE(TO_DATE(TRUNC(SYSDATE + 6469), 'yyyy/mm/dd')); -- Dec 31, 2031
  SYS.DBMS_OUTPUT.PUT_LINE(TO_DATE(TRUNC(SYSDATE - 4853), 'yyyy/mm/dd'));  -- Jan 01, 2001
  SYS.DBMS_OUTPUT.PUT_LINE(TO_DATE(TRUNC(SYSDATE + 7000), 'yyyy/mm/dd'));  -- ERROR
END

Simply put, when I add enough days to the SYSTEM date to exceed Dec 31, 2031 I get the following error message ... "a non-numeric character was found where a numeric was expected" ... . I experience the same problem when I subtract enough days from the SYSTEM date to pre-date Jan 01, 2001.

When I remove the format it's doesn't break but returns an impossible date ...

BEGIN
    -- returns 15-JUN-33  
    -- June 33rd 2015!!
    SYS.DBMS_OUTPUT.PUT_LINE(TO_DATE(TRUNC(SYSDATE + 7000))); 
END

So it appears to me that there is some sort of date range limit on this function ... JAN 01, 2001 - DEC 31, 2031 ... Is that true or am I doing something wrong here?

Thanks!

Answer 1

Let's work through the data types here.

sysdate returns a date . Adding a number to a date returns a date that many days in the future. So, for example, sysdate + 7000 is June 15, 2033. trunc(sysdate + 7000) also returns a date, it just sets the time to midnight. So far, so good.

The problem comes when you take that date and pass it to the to_date function. Logically, that doesn't make sense. You already have a date , there is no need to convert it to a date . Practically, to_date does not accept a date as a parameter. It only accepts a string. Now, Oracle can implicitly convert the date you have to a string using your session's nls_date_format setting which is what it does here. Best case, you're taking a date , implicitly converting that to a string, then explicitly converting that string back to exactly the same date that you started with. If your session's nls_date_format happens not to match the format mask that you're providing to the to_date , however, you'll likely get an error which is what you're seeing here.

Walking through an example, let's use the date of midnight on June 15, 2033. If you call to_date on that, Oracle has to convert the date to a string using your session's nls_date_format . If you're in the United States and you haven't changed anything about your client, your nls_date_format is probably DD-MON-RR . That means that your date gets converted to the string 15-JUN-33 when it is passed in to to_date . So, logically, you're trying to do something like

dbms_output.put_line( to_date( '15-JUN-33', 'yyyy/mm/dd' ));

When you look at it this way, it's obvious that the format mask doesn't match the format of the string which causes an error. If your nls_date_format is closer to the format mask in your to_date , it is possible that the to_date call will run successfully but return a different date than you expect (switching the month and the day for example).

The simple answer is that you should never call to_date on a day. You should only call to_date on a string. If you want to convert a date into a string in a particular format for display, use to_char not to_date .

Going back to the original question, yes there are limits to what constitutes a valid date in Oracle. A valid date must be between Jan 1, 4712 BC (6700 years ago) to Dec 31, 9999 (7900 years from now). It doesn't appear that you are anywhere near exceeding those limits.