August 18, 2009 at 5:04 pm
Comments posted to this topic are about the item When will the next 13th fall on a Friday?
Thanks,
Ashka Modi
Software Engineer || credEcard Technologies (india) Pvt. Ltd.
August 27, 2009 at 12:38 am
How about set-based approach:
SET DATEFIRST 1
;WITH days(Date) AS
(
--Get 13th of this month
SELECT CAST(CAST(YEAR(GETDATE()) AS VARCHAR(4)) + '-' + CAST(MONTH(GETDATE()) AS VARCHAR(2)) + '-13' AS DATETIME)
UNION ALL
--Get 13th of next months
SELECT DATEADD(MONTH, 1, Date) FROM days
)
SELECT TOP 1 * FROM days WHERE DATEPART(dw, Date) = 5 AND Date > GETDATE()
August 27, 2009 at 2:52 am
Good one! 🙂 but I am not much comfortable with UNION ALL. what you say?
Thanks,
Ashka Modi
Software Engineer || credEcard Technologies (india) Pvt. Ltd.
August 27, 2009 at 3:59 am
"UNION ALL" is a requirement in CTE
August 27, 2009 at 4:20 am
SET DATEFIRST 1
;WITH days(Date) AS
(
--Get 13th of this month
SELECT CAST(CAST(YEAR(GETDATE()) AS VARCHAR(4)) + '-' + CAST(MONTH(GETDATE()) AS VARCHAR(2)) + '-13' AS DATETIME)
UNION ALL
--Get 13th of next months
SELECT DATEADD(MONTH, 1, Date) FROM days
)
SELECT * FROM days
SELECT TOP 1 * FROM days WHERE DATEPART(dw, Date) = 5 AND Date > GETDATE()
Have you check this? check Execution plan also.
Thanks,
Ashka Modi
Software Engineer || credEcard Technologies (india) Pvt. Ltd.
August 27, 2009 at 4:48 am
I'm not sure what you mean.
"SELECT * FROM days" will fail because of recursion limit. But original idea was to get only the first Friday, the 13th. As there is always at least one such day in a year, there will be no more than 23 iterations.
Is there something wrong with execution plan?
Sorry for asking, but are you familiar with common table expressions (CTE)?
August 27, 2009 at 5:18 am
Another set based solution, this time using the good old Tally table[/url]. ((c) Jeff Moden et al. 19xx-2009)
SELECT TOP 1
DATEADD(dd,N-1,GETDATE())
FROM
Tally
WHERE
n < 366 --== One years worth of days
AND
DAY(DATEADD(dd,N-1,GETDATE())) = 13
AND
--==
--== Using @@DATEFIRST and the modulus operator we don't need to change or
--== assume anything about the current DATEFIRST setting
--==
(@@DATEFIRST + DATEPART(dw,DATEADD(dd,N-1,GETDATE()))) %7 = 6
ORDER BY
N
The execution plan for this indicates 100% of the query is spent doing a clustered index seek (nice!). That's if you build your tally table with a clustered index of course, as demonstrated in Jeff's tally table article.
And, as I indicated in the comments, you don't need to mess about with the DATEFIRST setting.
August 27, 2009 at 8:29 am
Rimvydas Gurskis (8/27/2009)
How about set-based approach:
SET DATEFIRST 1
;WITH days(Date) AS
(
--Get 13th of this month
SELECT CAST(CAST(YEAR(GETDATE()) AS VARCHAR(4)) + '-' + CAST(MONTH(GETDATE()) AS VARCHAR(2)) + '-13' AS DATETIME)
UNION ALL
--Get 13th of next months
SELECT DATEADD(MONTH, 1, Date) FROM days
)
SELECT TOP 1 * FROM days WHERE DATEPART(dw, Date) = 5 AND Date > GETDATE()
Correct me if I'm wrong, but that appears to be a recursive CTE .... which ISN'T set based at all. In fact, recurrsive CTE's are sometimes slower than a memory only While Loop.
--Jeff Moden
Change is inevitable... Change for the better is not.
August 27, 2009 at 11:04 am
I agree with Rimvydas' approach.
And for performance concerns on CTEs on this particular problem, this gives all the years from 1753 to 9999 that have 3 Friday the thirteenths.
It runs in about 2 to 3 seconds on my small test machine.
with thirteen (thedate) as(
select cast('01/13/1753' as datetime) as thedate
union all
select DATEADD(mm, 1, thedate)
from thirteen
where thedate < '01/01/9999'
)
select year(thedate) yr,
count(*) cnt
from thirteen
where DATENAME(dw, thedate) = 'Friday'
group by year(thedate)
having count(*) = 3
OPTION (MAXRECURSION 0)
August 28, 2009 at 2:16 am
Bill Coale (8/27/2009)
I agree with Rimvydas' approach.And for performance concerns on CTEs on this particular problem, this gives all the years from 1753 to 9999 that have 3 Friday the thirteenths.
It runs in about 2 to 3 seconds on my small test machine.
If your concerned with performance I'd go for the tally table approach, The following executes in around 100ms on my machine.
SELECT
YEAR(DATEADD(mm,N-1,'17530113')),
COUNT(*)
FROM
dbo.Tally
WHERE
N < DATEDIFF(mm,'17530113','99990101')
AND
DATENAME(dw, DATEADD(mm,N-1,'17530113')) = 'Friday'
GROUP BY
YEAR(DATEADD(mm,N-1,'17530113'))
HAVING
count(*) = 3
ORDER BY
YEAR(DATEADD(mm,N-1,'17530113'))
August 28, 2009 at 2:27 am
An this is slightly faster again, by avoiding the string comparison ( = 'Friday'):
SELECT
YEAR(DATEADD(mm,N-1,'17530113')),
COUNT(*)
FROM
dbo.Tally
WHERE
N < DATEDIFF(mm,'17530113','99990101')
AND
(DATEPART(dw,DATEADD(mm,N-1,'17530113')) + @@DATEFIRST) % 7 = 6
GROUP BY
YEAR(DATEADD(mm,N-1,'17530113'))
HAVING
count(*) = 3
ORDER BY
YEAR(DATEADD(mm,N-1,'17530113'))
August 28, 2009 at 7:13 am
Nigel,
Yes, your tally approach is much faster. 100 ms vs 1400 ms on my machine.
Thanks,
Bill
September 10, 2009 at 12:12 am
If the current date is equal to 13 then your query will give that as Friday.
IF (DATEPART(dd, @currDate) 13)
what about adding one more condition with above one
OR (DATEPART(dw, @currDate) 1)
then I think your query is perfect.
🙂
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