Re: [sqlite] Converting Daily to Weekly

[Igor Tandetnik]

On 2/3/2012 12:45 PM, rixtertrader wrote:

Igor Tandetnik wrote:

It could be written more explicitly:

GROUP BY Year + (case WeekNum when 53 then 1 else 0 end),
 (case WeekNum when 53 then 1 else WeekNum end);


Okay, I get it. GROUP by Year and if week 53 it becomes part of the next
Year. GROUP by WeekNum and if 53 it becomes 1.

Does this effect the Max(High) and Max(Low) values as well? In other words,
will the values of week 53 be part of the equation with week 1 when
returning Max(High) and Max(Low) for the week?


Yes. All rows for which expressions in the GROUP BY clause evalulate to 
the same values end up in the same group. Aggregate functions then work 
on that group.

--
Igor Tandetnik

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Re: [sqlite] Converting Daily to Weekly

[rixtertrader]

Igor Tandetnik wrote:
> 
> It's simple, really. (WeekNum=53) is a boolean expression, which has a
> value of 1 (for true) or 0 (for false). So Year + (WeekNum=53) is equal to
> Year most of the time, but Year + 1 when WeekNum is 53. Similarly WeekNum 
> - (WeekNum=53)*52 is equal to WeekNum most of the time, but to 1 (53 - 1 *
> 52) when WeekNum is 53.
> 
> It could be written more explicitly:
> 
> GROUP BY Year + (case WeekNum when 53 then 1 else 0 end), 
> (case WeekNum when 53 then 1 else WeekNum end);
> 
> 

Okay, I get it. GROUP by Year and if week 53 it becomes part of the next
Year. GROUP by WeekNum and if 53 it becomes 1.

Does this effect the Max(High) and Max(Low) values as well? In other words,
will the values of week 53 be part of the equation with week 1 when
returning Max(High) and Max(Low) for the week?

Thanks for your replies. Very educational!

:)

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Re: [sqlite] Converting Daily to Weekly

[Igor Tandetnik]

rixtertrader  wrote:
> Igor Tandetnik wrote:
>> GROUP BY Year + (WeekNum=53), WeekNum  - (WeekNum=53)*52
>> 
>> This should effectively merge week 53 with week 1 of next year.
> 
> I don't understand how it works.

It's simple, really. (WeekNum=53) is a boolean expression, which has a value of 
1 (for true) or 0 (for false). So Year + (WeekNum=53) is equal to Year most of 
the time, but Year + 1 when WeekNum is 53. Similarly WeekNum  - (WeekNum=53)*52 
is equal to WeekNum most of the time, but to 1 (53 - 1 * 52) when WeekNum is 53.

It could be written more explicitly:

GROUP BY Year + (case WeekNum when 53 then 1 else 0 end), 
(case WeekNum when 53 then 1 else WeekNum end);

Perhaps I was being too clever.
-- 
Igor Tandetnik

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Re: [sqlite] Converting Daily to Weekly

[rixtertrader]

Igor Tandetnik wrote:
> 
> rixtertrader  wrote:
>> Here is the SQL string I'm currently using that has this problem:
>> 
>> SELECT date(max(Date), 'weekday 5') as MaxDate, Date, Year, Month, Week,
>> Max(High) as High, Min(Low) as Low, Week as WeekNum FROM [" & sTable & "]
>> GROUP By Year, Week
> 
> Try
> 
> GROUP BY Year + (WeekNum=53), WeekNum  - (WeekNum=53)*52
> 
> This should effectively merge week 53 with week 1 of next year.
> -- 
> Igor Tandetnik
> 
> ___
> 


That was absolutely brilliant. Thank you.

I don't understand how it works. I'm sure some of that has to do with the
fact that I'm a 1 out of 10 when it comes to understanding SQL, and I wrote
this so long ago. 

I don't suppose this can be explained to me in a dumbed-down sort of way. 
:-/


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Re: [sqlite] Converting Daily to Weekly

[Igor Tandetnik]

rixtertrader  wrote:
> Here is the SQL string I'm currently using that has this problem:
> 
> SELECT date(max(Date), 'weekday 5') as MaxDate, Date, Year, Month, Week,
> Max(High) as High, Min(Low) as Low, Week as WeekNum FROM [" & sTable & "]
> GROUP By Year, Week

Try

GROUP BY Year + (WeekNum=53), WeekNum  - (WeekNum=53)*52

This should effectively merge week 53 with week 1 of next year.
-- 
Igor Tandetnik

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Re: [sqlite] Converting Daily to Weekly

[Petite Abeille]

On Feb 2, 2012, at 9:30 PM, rixtertrader wrote:

> This is because the year started in the middle of the first week.

Perhaps you might be interested in using the ISO week instead:

http://en.wikipedia.org/wiki/ISO_week_date

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