Pintér Tibor wrote:
>
> Ananda Kumar írta:
>> Hi All,
>> I think i worked on this and found the results.
>>
>>
>> I did the below.
>>
>> 1. Multiplied the week_of_the_year with 7 (7 days per week), to get the
>> total number of days from begning of the year.
>>
>> 2. used mysql function makedate
>> makedate(year,number of days from the start of the year)
>> makedate(2008,224)
>> select makedate(2008,224);
>> +--------------------+
>> | makedate(2008,224) |
>> +--------------------+
>> | 2008-08-11 |
>
> this is definitely wrong, since you dont care about the fact that the
> frist day of the year is not always Monday
I agree it is wrong, but for a slightly different reason:
What is the definition of "week of year" ?
One problem is that the weekday of January 1 varies, the other is that
the definition of "week" may not be universal (does it start with Sunday
or Monday ?).
I know of one widespread definition that (AFAIR) is (loosely)
"The first week which has more than half of its days in a given year is
called 'week 1' of that year."
If you take Sunday as the start of the week, this translates to
"Week 1 is the week which contains the first Wednesday of a year."
(If your week starts Monday, the first Thursday determines it.)
There is another definition that (loosely) says
"The first week which has all its days in a given year is called 'week
1' of that year."
Again, it is a separate question whether your weeks start Sunday or Monday.
By both definitions, January 1 need not belong to week 1, it may belong
to the last week of the previous year.
See these lines quoted from Linux "man date":
> ~> man date | grep week
> ...
> %g last two digits of year of ISO week number (see %G)
> %G year of ISO week number (see %V); normally useful only with %V
> %u day of week (1..7); 1 is Monday
> %U week number of year, with Sunday as first day of week (00..53)
> %V ISO week number, with Monday as first day of week (01..53)
> %w day of week (0..6); 0 is Sunday
> %W week number of year, with Monday as first day of week (00..53)
According to Stevens ("Advanced Programming in the Unix environment",
page 158), the "%U" and "%W" codes seem to use the "all days" (second)
definition.
I *guess* that the separate "ISO" reference implies that the ISO
definition uses the "more than half" (first) definition, but I propose
you check yourself. From some references, I take it that is ISO 8601.
To return to the original question:
Sorry, I do not know a MySQL function to do that mapping.
Your application language might offer something:
From C, "strftime()" and/or "strptime()" might help.
From Perl, I assume you can find something in CPAN.
From other languages, I have no idea off-hand.
But before coding anything, you have to check your definition of "week
number", there are several to choose from.
Regards,
Jörg
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Joerg Bruehe, MySQL Build Team, joerg@stripped (+49 30) 417 01 487
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