Calculate inactive customers from single table - sql

Question

I have table with fields Customer.No. , Posting date, Order_ID . I want to find total inactive customers for last 12 months on month basis which means they have placed order before 12 months back and became in active. So want calculate this every month basis to under stand how inactive customers are growing month by month.
if I run the query in July it should go back 365 days from the previous month end and give total number of inactive customers. I want to do this month by month.
I am in learning stage please help.
Thanks for your time in advance.

Answer 1

to get the customers
SELECT DISTINCT a.CustomerNo
FROM YourTable a
WHERE NOT EXISTS
(SELECT 0 FROM YourTable b WHere a.CustomerNo = b.CustomerNo
and b.PostingDate >
dateadd(day,-365 -datepart(day,getdate()),getdate())
)
to get a count
SELECT DISTINCT count(0) as InnactiveCount
FROM YourTable a
WHERE NOT EXISTS
(SELECT 0 FROM YourTable b WHere a.CustomerNo = b.CustomerNo
and b.PostingDate >
dateadd(day,-365 -datepart(day,getdate()),getdate())
..
generate a 'months' table by CTE, then look for inactive in those months
;WITH month_gen as (SELECT dateadd(day,-0 -datepart(day,getdate()),getdate()) eom, 1 as x
UNION ALL
SELECT dateadd(day,-datepart(day,eom),eom) eom, x + 1 x FROM month_gen where x < 12
)
SELECT DISTINCT CONVERT(varchar(7), month_gen.eom, 102), count(0) innactiveCount FROM YourTable a
cross join month_gen
WHERE NOT EXISTS(SELECT 0 FROM YourTable b WHere a.CustomerNo = b.CustomerNo and
YEAR(b.PostingDate) = YEAR(eom) and
MONTH(b.PostingDate) = MONTH(eom)
)
GROUP BY CONVERT(varchar(7), month_gen.eom, 102)
if that gets you anywhere, maybe a final step is to filter out anything getting 'counted' before it was ever active i.e. don't count 'new' customers before they became active

Answer 2

Try below query. To achieve your goal you need calendar table (which I defined with CTE). Below query counts inactivity for the first day of a month:
declare #tbl table (custNumber int, postDate date, orderId int);
insert into #tbl values
(1, '2017-01-01', 123),
(2, '2017-02-01', 124),
(3, '2017-02-01', 125),
(1, '2018-02-02', 126),
(2, '2018-05-01', 127),
(3, '2018-06-01', 128)
;with cte as (
select cast('2018-01-01' as date) dt
union all
select dateadd(month, 1, dt) from cte
where dt < '2018-12-01'
)
select dt, sum(case when t2.custNumber is null then 1 else 0 end)
from cte c
left join #tbl t1 on dateadd(year, -1, c.dt) >= t1.postDate
left join #tbl t2 on t2.postDate > dateadd(year, -1, c.dt) and t2.postDate <= c.dt and t1.custNumber = t2.custNumber
group by dt