add missing month in sales - sql

Question

I have a sales table with below values.
TransactionDate,CustomerID,Quantity
2020-01-01,1234,5
2020-07-01,1234,9
2020-03-01,3241,8
2020-07-01,3241,4
As you can see first purchase was for CustomerID = 1234 in Jan 2020 and for CustomerID = 3241 in MAR 2020.
I want on output where in all the date should be filled up with 0 purchase value.
means if there is no sale between Jan and July Then output should be as below.
TransactionDate,CustomerID,Quantity
2020-01-01,1234,5
2020-02-01,1234,0
2020-03-01,1234,0
2020-04-01,1234,0
2020-05-01,1234,0
2020-06-01,1234,0
2020-07-01,1234,9
2020-03-01,3241,8
2020-04-01,3241,0
2020-05-01,3241,0
2020-06-01,3241,0
2020-07-01,3241,4

Answer 1

You can use a recursive query to create the missing dates per customer.
with recursive dates (customerid, transactiondate, max_transactiondate) as
(
select customerid, min(transactiondate), max(transactiondate)
from sales
group by customerid
union all
select customerid, dateadd(month, 1, transactiondate), max_transactiondate
from dates
where transactiondate < max_transactiondate
)
select
d.customerid,
d.transactiondate,
coalesce(s.quantity, 0) as quantity
from dates d
left join sales s on s.customerid = d.customerid and s.transactiondate = d.transactiondate
order by d.customerid, d.transactiondate;

Answer 2

This is a convenient place to use a recursive CTE. Assuming all your dates are on the first of the month:
with cr as (
select customerid, min(transactiondate) as mindate, max(transactiondate) as maxdate
from t
group by customerid
union all
select customerid, dateadd(month, 1, mindate), maxdate
from cr
where mindate < maxdate
)
select cr.customerid, cr.mindate as transactiondate, coalesce(t.quantity, 0) as quantity
from cr left join
t
on cr.customerid = t.customerid and
cr.mindate = t.transactiondate;
Here is a db<>fiddle.
Note that if you have more than 100 months to fill in, then you will need option (maxrecursion 0).
Also, this can easily be adapted if the dates are not all on the first of the month. But you would need to explain what the result set should look like in that case.

Answer 3

[EDIT] Based on what other posted I updated the code.
;with
min_date_cte(MinTransactionDate, MaxTransactionDate) as (
select min(TransactionDate), max(TransactionDate) from tsales),
unq_yrs_cte(year_int) as (
select distinct year(TransactionDate) from tsales),
unq_cust_cte(CustomerID) as (
select distinct CustomerID from tsales)
select datefromparts(uyc.year_int, v.month_int, 1) TransactionDate,
ucc.CustomerID,
isnull(t.Quantity, 0) Quantity
from min_date_cte mdc
cross join unq_yrs_cte uyc
cross join unq_cust_cte ucc
cross join (values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12)) v(month_int)
left join tsales t on datefromparts(uyc.year_int, v.month_int, 1)=t.TransactionDate
and ucc.CustomerID=t.CustomerId
where
datefromparts(uyc.year_int, v.month_int, 1)>=mdc.MinTransactionDate
and datefromparts(uyc.year_int, v.month_int, 1)<=mdc.MaxTransactionDate;
Results
TransactionDate CustomerID Quantity
2020-01-01 1234 5
2020-01-01 3241 0
2020-02-01 1234 0
2020-02-01 3241 0
2020-03-01 1234 0
2020-03-01 3241 8
2020-04-01 1234 0
2020-04-01 3241 0
2020-05-01 1234 0
2020-05-01 3241 0
2020-06-01 1234 0
2020-06-01 3241 0
2020-07-01 1234 9
2020-07-01 3241 4

Answer 4

You can make use of recursive query:
WITH cte1 as
(
select customerid, min([TransactionDate]) as Monthly_date, max([TransactionDate]) as end_date from calender_table
group by customerid
union all
select customerid, dateadd(month, 1, Monthly_date), end_date from cte1
where Monthly_date < end_date
)
select a.Monthly_date, a.customerid,coalesce(b.quantity, 0) from cte1 a left outer join calender_table b
on (a.Monthly_date = b.[TransactionDate] and a.customerid = b.customerid)
order by a.customerid, a.Monthly_date;