Oracle returning incorrect day of the week

Question

Hi there,
I have a table named Performance, which includes a date field named PDATE, some other attributes such as title, etc, and a field named WEEKDAY, which includes the numerical value for the day of the week. Here is an example:
PER#
P#
THEATRE#
WEEKDAY
PDATE
PHOUR
PMINUTE
COMMENTS
1
1
1
1
02-MAR-10
19
30
Normal evening time
2
1
1
2
03-MAR-10
19
30
Normal evening time
58
6
6
3
04-MAR-10
19
30
Normal evening time
5
1
1
4
05-MAR-10
19
30
Normal evening time
I went to verify the day of the week in a calendar (in fact I checked more than one) and I noticed that the Weekday value seems incorrect. For instance, the 2nd of March was a Tuesday so I would expect WEEKDAY to say 3 (considering Sunday to be the 1st day of the week, which is the universal norm), or perhaps 2 (considering Monday to be the 1st day of the week).
First thing that came to mind is, since the WEEKDAY is a number field which has been pre-populated, the person who populated it made a mistake.
So I went to verify this by running a query that would give me the correct day of the week by extracting it from the PDATE field:
SELECT DISTINCT pdate, EXTRACT(year from pdate) year,
to_char(to_date(pdate,'DD/MM/YYYY'),'DY') weekdaycalc, weekday
FROM ops$yyang00.Performance
ORDER BY pdate
And to my astonishment, I got the following table as a return:
PDATE
YEAR
WEEKDAYCALC
WEEKDAY
02-MAR-10
2010
SUN
1
03-MAR-10
2010
MON
2
04-MAR-10
2010
TUE
3
05-MAR-10
2010
WED
4
06-MAR-10
2010
THU
5
07-MAR-10
2010
FRI
6
08-MAR-10
2010
SAT
7
09-MAR-10
2010
SUN
1
10-MAR-10
2010
MON
2
As you can see, all of the above WEEKDAYCALC values are incorrect, and seem to match the incorrect values in WEEKDAY.
I could perhaps understand some weird WEEKDAY numbering if there is some general database setting where I could specify any day of the week to be the first day. However, I can't think of any logical explanation on why Oracle would call a Tuesday Sunday.
Could someone please advise what I mght be doing wrong? This is driving me insane!
Thank you in advance for all the help and support.
Regards,
P.

Answer 1

Hi,
1bded5c7-e555-4306-ac86-45da83dff439 wrote:
Thanks both Frank and Solomon for the help. I am quite new to Oracle and I feel you folks went the extra mile on the explanation
Based on your explanations, I changed the formula so the date format would be processed as YY rather than YYY.
SELECT DISTINCT pdate, EXTRACT(year from pdate) year,   
to_char(to_date(pdate,'DD/MM/YY'),'DY') weekdaycalc, weekday   
FROM Performance   
ORDER BY pdate  
And I got the correct values now:
PDATE
YEAR
WEEKDAYCALC
WEEKDAY
02-MAR-10
2010
TUE
1
03-MAR-10
2010
WED
2
04-MAR-10
2010
THU
3
05-MAR-10
2010
FRI
4
06-MAR-10
2010
SAT
5
07-MAR-10
2010
SUN
6
08-MAR-10
2010
MON
7
The interesting bit here, is the original WEEKDAY field, which clearly has been created with the same mistake I did (this whole table wasn't created by me). I reckon I spotted a design bug.
Regards,
P.
No; you're still calling TO_DATE on something that is already a DATE, and you're still using an implicit conversion (only now you're using a format such that the current NLS settings don't cause you to get punished.  If your DBA changes the NLS settings next week, then you will have similar errors again.)
If you want to see what day of the week pdate is, then use:
TO_CHAR (pdate, 'DY')
or, to make it NLS-independent:
TO_CHAR (pdate, 'DY', 'NLS_DATE_LANGUAGE=ENGLISH')
This is simpler than what you posted, it's more efficient and easier to debug and to maintain, and it also gets the results you want in all cases.
One more time: TO_DATE, as the name hints, converts something (a VARCHAR2, to be precise) TO a DATE.  If pdate is already a DATE, then what do you need to convert?  You're implicitly converting a DATE to a VARCHAR2, then explicitly converting that VARCHAR2 back to a DATE, and then calling TO_CHAR.  That's 3 function calls (1 of them error prone) where only 1 is needed.
Also if you ever have to reconcile 2-digit years with 4-digit years (and, again, in this problem you don't) the correct way is to convert the 2-digit year to a 4-digit year, not vice-versa.
Calling Tuesday the 1st day of the week could make sense in a particular application.