Find missing months in SQL - sql

Question

So this post remains unanswered and not useful
Finding missing month from my table
This link Get Missing Month from table requires a lookup table... which is not my first choice.
I have a table with Financial Periods, and a reference number. Each reference numbers has a series of financial periods which may start anywhere, and end anywhere. The test is simply that between the start and end, there is no gap - i.e. there must be every financial period period the smallest and largest dates, when grouped by reference number.
A financial period is a month.
So... in this example below, Reference Number A is missing May 2016.
REF MONTH
A 2016-04-01
A 2016-06-01
A 2016-07-01
B 2016-03-01
B 2016-04-01
B 2016-05-01
C 2022-05-01
-- Find the boundaries of each ref
select REF
, MIN(Month) as smallest
, MAX(Month) as largest
from myTable
group by REF
-- But how to find missing items?
SQL Server 2019.

Answer 1

Clearly a Calendar Table would make this a small task (among many others)
Here is an alternative using the window function lead() over()
Example
Declare #YourTable Table ([REF] varchar(50),[MONTH] date) Insert Into #YourTable Values
('A','2016-04-01')
,('A','2016-06-01')
,('A','2016-07-01')
,('B','2016-03-01')
,('B','2016-04-01')
,('B','2016-05-01')
,('C','2022-05-01')
;with cte as (
Select *
,Missing = datediff(MONTH,[Month],lead([Month],1) over (partition by Ref order by [Month]))-1
From #YourTable
)
Select * from cte where Missing>0
Results
REF MONTH Missing
A 2016-04-01 1

Answer 2

I added one more row of input to demonstrate the solution better.
with forecast (
REF,
[MONTH]
) as (
select REF
, [MONTH]
from (
values
('A', {d '2016-04-01'})
, ('A', {d '2016-06-01'})
, ('A', {d '2016-07-01'})
, ('B', {d '2016-03-01'})
, ('B', {d '2016-04-01'})
, ('B', {d '2016-05-01'})
, ('B', {d '2016-09-01'})
, ('C', {d '2022-05-01'})
) x (REF, [MONTH])
),
-- define the date ranges
daterange as (
select REF
, min([MONTH]) as dtmin
, max([MONTH]) as dtmax
from forecast
group by REF
),
-- get all of the [end of month] dates in the range
dt (
REF,
[MONTH]
) as (
select REF
, dtmin
from daterange dr
union all
select dt.REF
, dateadd(month, 1, [MONTH])
from dt dt
inner join daterange dr on dr.REF = dt.REF
where dateadd(month, 1, [MONTH]) <= dr.dtmax
)
-- find the missing months
select REF
, [MONTH]
from dt
except
select REF
, [MONTH]
from forecast
order by 1, 2
-- or list all of the months for each REF
--select REF
--, [MONTH]
--from dt