TSQL loop months in sequence - sql-server

Question

I have an query that I'm feeling out-of-my depth with.
I need to loop through months between two dates and return a subset of data for each month with a blank row for months with no data.
For example:
TransactionID | Date | Value
1 | 01/01/2015 | £10
2 | 16/01/2015 | £15
3 | 21/01/2015 | £5
4 | 15/03/2015 | £20
5 | 12/03/2015 | £15
6 | 23/04/2015 | £10
Needs to return:
Month | Amount
January | £30
February | £0
March | £35
April | £10
My query will rely on specifying a date range so I can set the first and last date of the query.
I feel like I maybe over thinking this, but have gotten to that stage where you start to feel like you tying yourself in knots.

Answer 1

The key is having access to a list of integers to represent the months in the range. If you don't have a Numbers Table, then spt_values will do in a pinch.
SqlFiddle Demo
SELECT
[Year] = YEAR(DATEADD(month,[i],#range_start))
,[Month] = DATENAME(month,DATEADD(month,[i],#range_start))
,[Amount] = ISNULL(SUM([Value]),0)
FROM (
SELECT TOP (DATEDIFF(month,#range_start,#range_end)+1)
ROW_NUMBER() OVER(ORDER BY (SELECT 1))-1 [i]
FROM master.dbo.spt_values
) t1
LEFT JOIN #MyTable t2
ON (t1.[i] = DATEDIFF(month,#range_start,t2.[Date]) )
GROUP BY [i]
ORDER BY [i]

Answer 2

SQL is a tricky language at first. You actually do not want a loop. In fact, you pretty much never want to loop in SQL except in very few cases. Try this out:
DECLARE #StartDate DATE,
#EndDate DATE;
SET #StartDate = '01 January 2015';
SET #EndDate = '30 April 2015';
WITH CTE_Months
AS
(
SELECT #StartDate dates
UNION ALL
SELECT DATEADD(MONTH,1,dates)
FROM CTE_Months
WHERE DATEADD(MONTH,1,dates) < #EndDate
)
SELECT YEAR(B.[date]) AS yr,
DATENAME(MONTH,B.[Date]) AS month_name,
SUM(ISNULL(B.Value,0)) AS Amount
FROM CTE_Months A
LEFT JOIN yourTable B
ON YEAR(A.[date]) = YEAR(B.[date])
AND MONTH(A.[date]) = MONTH(B.[date])
GROUP BY YEAR(B.[date]),DATENAME(MONTH,B.[Date])

Answer 3

One way: create a table called months with a monthnum int field and 12 rows of [1..12]
declare #start date = '01 jan 2015',
#end date = '30 apr 2015'
select
datename(month, dateadd(month, monthnum, 0) - 1),
isnull(Amount, 0)
from months
left join (
select
month(date) Month,
sum(Value) Amount
from tbl
where date between #start and #end
group by month(date)
) T on (T.Month = months.monthnum)
where months.monthnum between month(#start) and month(#end)
order by monthnum

Answer 4

The following code will generate one output row for each month between the first and last transaction dates. Spanning a year boundary, or multiple years, is handled correctly.
-- Some sample data.
declare #Transactions as Table
( TransactionId Int Identity, TransactionDate Date, Value Int );
insert into #Transactions ( TransactionDate, Value ) values
( '20141125', 10 ), ( '20150311', 20 ), ( '20150315', 5 ), ( '20150509', 42 );
select * from #Transactions;
with
-- Determine the first and last dates involved.
Range as (
select Min( TransactionDate ) as FirstDate, Max( TransactionDate ) as LastDate
from #Transactions ),
-- Generate a set of all of the months in the range.
Months as (
select DateAdd( month, DateDiff( month, 0, FirstDate ), 0 ) as Month,
DateAdd( month, DateDiff( month, 0, LastDate ), 0 ) as LastMonth
from Range
union all
select DateAdd( month, 1, Month ), LastMonth
from Months
where Month < LastMonth )
-- Summarize the transactions.
select M.Month, Coalesce( Sum( T.Value ), 0 ) as Total
from Months as M left outer join
#Transactions as T on DateAdd( month, DateDiff( month, 0, T.TransactionDate ), 0 ) = M.Month
group by M.Month
order by M.Month
option ( MaxRecursion 1000 );