First day of the give Month and Year

Hello,
   I have a this stored procedure to which the year and month Number is pass as integer
   input.
   
For the SQL within that stored procedure, using the input month and Year (which is in integer data type), I need to derive a DATE as first day of that input month and year.

Eg:
CREATE PROCEDURE test (@i_Year INT, @i_Month INT)
BEGIN

  DECLARE @v_date DATE;
  SET @v_date = <need to assign 1st day of the input month/year i.e, '20140101'>

END

Input values passed to the stored procedure:
@i_Year = 2014
@i_Month = 1

Below is what I figured out:
SELECT 
CONVERT(DATE,     (CONVERT(VARCHAR,@v_Year)+CONVERT(VARCHAR,@v_Month)+'01')
               )

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But is there a better way to achieve this?

Is there a date function which would take month and year, and return me the first day in that month/year, and also the value should be of type DATE?

Thanks!
sath350163Asked:
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David ToddConnect With a Mentor Senior DBACommented:
Hi,

Howabout

declare @year int
declare @month int

set @year = 2011
set @month = 7

select dateadd( month, @month - 1, dateadd( year, @year - 1900, 0 ))

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HTH
  David
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Olaf DoschkeSoftware DeveloperCommented:
ISO date literals, eg '20140101' can be used - without CONVERT. There's implicit conversion done, so this works:

declare @v_date as date;

set @v_date ='20140301'

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Change parameters to char(4) and char(2) or convert @iYear*100+@i_Month to a char(6) to concatenate '01'.

Bye, Olaf.

edit: ... or create a char(8) for implicit conversion:
set @v_date = convert(char(8),@i_Year*10000+@i_Month*100+1)

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This just won't work with years before 1000, you'd have to add leading zeros, then.

Bye, Olaf.
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David ToddSenior DBACommented:
Hi,

If you have a datetime, the easiest, fastest way is to keep it as a date time like this:

select dateadd( year, datediff( year, 0, getdate()), 0 )

Works for years, quarters, months, days, hours, minutes.

HTH
  David

PS converting to/from chars is slow - okay for a one-off, but not so much when grouping a bunch of transactions.
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Olaf DoschkeConnect With a Mentor Software DeveloperCommented:
> dateadd( year, datediff( year, 0, getdate()), 0 )
That gives the first day of the current year.

dateadd( month, datediff( month, 0, getdate()), 0 )
gives the first day of the current month.

That's fine for these special cases, but no answer for the parameterisation of @i_Year, @i_Month.

So, Dave, you should explain 0 is taken as SQL Server default date 1900-01-01, so you could use the number of months to add to january, 1st 1900:

Select dateadd( month, (@i_Year-1900)*12+@i_Month-1, 0 )

eg:

declare @i_Year int, @i_Month int

set @i_Year = 2011
set @i_Month = 6

Select dateadd( month, (@i_Year-1900)*12+@i_Month-1, 0 )

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Bye, Olaf.

Edit: I just did a little benchmark. Turns out dateadd is faster, that's no surprise. But it's just taking 40% of the char conversion.

I'm a bit surprised how much faster that is, actually. I wouldn't have thought a dateadd solution is more than twice as fast, even though it's two conversions (int->string->date) vs one date computation (dateadd) additional to a simple numeric computation, which would be neglectable in both cases.

declare @v_date date;
declare @t0 datetime;
declare @iJunk int;
declare @offset int;

use tempdb;
DECLARE @Dates table (iYear int, iMonth int);

Insert Into  @Dates
SELECT TOP 10000000 
RAND(checksum(newid()))*100+1950 as iYear, 
RAND(checksum(newid()))*12+1 as iMonth
FROM MASTER..spt_values a, MASTER..spt_values b;

--time to query all rows 
Select @t0=getdate();
Select @iJunk= iYear, @iJunk=iMonth from  @Dates;
--this is an offset time both of the following queries take just to query the data:
Select @offset = datediff(ms,@t0,getdate()); 
--subtract this time from both of the following results to get the net time for conversions.

--time to create a date with dateadd from 1900-01-01
Select @t0=getdate();
Select @v_date = dateadd( month, (iYear-1900)*12+iMonth-1, 0 ) from  @Dates;
Select datediff(ms,@t0,getdate())-@offset;

--time to create a date with conversion of int to char and implicit conversion of char to date
Select @t0=getdate();
Select @v_date = convert(char(8),iYear*10000+iMonth*100+1) from @Dates;
Select datediff(ms,@t0,getdate())-@offset;

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Olaf DoschkeSoftware DeveloperCommented:
Hi David,

of course that works, too. My benchmark says it's taking about two times as long, though. two vs one dateadd, quite logical.

So what's wrong with dateadd( month, (@i_Year-1900)*12+@i_Month-1, 0 )?

Too complicated?

Bye, Olaf.
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sath350163Author Commented:
Appreciate the response from both of you.

Both the below approaches works well for my use case.

Select dateadd( month, (@i_Year-1900)*12+@i_Month-1, 0 )

select dateadd( month, @month - 1, dateadd( year, @year - 1900, 0 ))

Thanks a lot!
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Olaf DoschkeSoftware DeveloperCommented:
Thanks a bunch,

you could take it one step further and have a date generation function with year,month,day by using

dateadd(day, @day-1, dateadd( month, @month - 1, dateadd( year, @year - 1900, 0 )))

In your sepcific case the @day always is 1 and so the outer dateadd isn't needed. It's much harder to replace that with a single dateadd on day basis, as you'd need to compute days per year and month, with leap years and the differing month lengths etc.

It's easy enough to do with months only, though, and instead of starting from 0 start from day-1. So a generic solution with @year, @month and @day parameters could also be

dateadd(month, (@year - 1900) * 12 + @month - 1 , @day - 1)

Which would compare to C# DateTime(Int32, Int32, Int32) for year,month, day ints.

It's a pity T-SQL doesn't offer that simple date function in the first place, which was your original good question.

Bye, Olaf.
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