Oracle weird behavior string comparison with day of the week

While the below code prints 'Wrong Thursday',(10-FEB is a Thursday)

BEGIN
  IF to_char(to_date('10-FEB-2011','DD-MON-YYYY'),'Day')='Thursday' THEN
    dbms_output.put_line('Correct');

  ELSE
    dbms_output.put_line('Wrong '||to_char(to_date('10-FEB-2011','DD-MON-YYYY'),'Day'));
  END IF;
END;

The following prints 'Correct',(09-FEB is a Wednesday)

BEGIN
  IF to_char(to_date('09-FEB-2011','DD-MON-YYYY'),'Day')='Wednesday' THEN
    dbms_output.put_line('Correct');

  ELSE
    dbms_output.put_lin开发者_Python百科e('Wrong '||to_char(to_date('09-FEB-2011','DD-MON-YYYY'),'Day'));
  END IF;
END;

I've been trying to figure this out but I couldn't. Any help would be much appreciated. Thanks in advance.

to_char by default is space-padded:

SQL> BEGIN
  2    dbms_output.put_line('x' ||
  3                         to_char(to_date('10-FEB-2011','DD-MON-YYYY'),'Day')||
  4                         'x');
  5  END;
  6  /

xThursday x

Use the modifier fm to prevent the padding:

SQL> BEGIN
  2    IF to_char(to_date('10-FEB-2011','DD-MON-YYYY'),'fmDay')='Thursday' THEN
  3      dbms_output.put_line('Correct');
  4    ELSE
  5      dbms_output.put_line('Wrong '||to_char(to_date('10-FEB-2011','DD-MON-YYYY'),'fmDay'));
  6    END IF;
  7  END;
  8  /

Correct

PL/SQL procedure successfully completed

Hint:

select length(to_char(to_date('10-FEB-2011','DD-MON-YYYY'),'Day')) from dual;

returns 9, although length('Thursday') is 8.

More information is found on Oracle's docu at http://download.oracle.com/docs/cd/E11882_01/server.112/e17118/sql_elements004.htm#i34510: The character elements MONTH, MON, DAY, and DY are padded with trailing blanks to the width of the longest full month name, the longest abbreviated month name, the longest full date name, or the longest abbreviated day name, respectively, among valid names determined by the values of NLS_DATE_LANGUAGE and NLS_CALENDAR parameters. For example, when NLS_DATE_LANGUAGE is AMERICAN and NLS_CALENDAR is GREGORIAN (the default), the largest element for MONTH is SEPTEMBER, so all values of the MONTH format element are padded to nine display characters. The values of the NLS_DATE_LANGUAGE and NLS_CALENDAR parameters are specified in the third argument to TO_CHAR and TO_* datetime functions or they are retrieved from the NLS environment of the current session.

Emphasis mine.

Another Hint: Try using a trim

All days of week will be length 9.