In SQL, is there a way to show all dates even if the date doesn't have data points? - sql

Question

I have a transaction table t as follows in MS SQL Management Studio:
If I run the following SQL to summarise the transaction:
Select
Format(Transaction_Date, 'MMM-yyyy') as 'Year/Month'
,Customer
,Count(Customer) as SalesCount
From t
Group by Format(Transaction_Date, 'MMM-yyyy'), Customer
Order by Customer, Format(Transaction_Date, 'MMM-yyyy')
I'll get:
However I was asked to add all the months for the year 2019 and if there's no transaction in a certain month then return 0 for the SalesCount column:
I tried to create a month table with all the months in 2019 and left join it with the transaction table, but it still returns the same result with no showing of the months without transactions.
Time table I created:
declare #StartDate date = '2019-01-01';
declare #EndDate date = '2020-01-01';
With cte as (
Select #StartDate AS myDate
Union All
Select Dateadd(Month,1,myDate)
From cte
Where Dateadd(Month,1,myDate) < #EndDate
)
,TimeTable as(
SELECT
year(myDate)
,Datename(Month,myDate)
,Format(myDate,'MMMM-yy') as 'Month-Year'
FROM cte
)
Select
tb.'Month-Year'
t.Format(Transaction_Date, 'MMM-yyyy') as Year/Month
,t.Customer
,t.Count(Customer) as SalesCount
From TimeTable tb
Left Join Transaction t on t.'Month/Year' = tb.'Month-Year'
Group by tb.'Month-Year', Format(Transaction_Date, 'MMM-yyyy'), Customer
Order by Customer, Format(Transaction_Date, 'MMM-yyyy')
Your help will be much appreciated!

Answer 1

You need to generate all the rows for the months. One method uses a recursive CTE. Then rest is then left join and aggregation.
Let me assume you are using SQL Server:
with months as (
select convert(date, '2019-01-01') as mon
union all
select dateadd(month, 1, mon)
from months
where mon < '2019-12-01'
)
select Format(m.mon, 'MMM-yyyy') as year_month,
c.Customer,
count(t.customer) as SalesCount
from months m cross join
(select distinct customer from t) c left join
t
on t.transaction_date >= m.mon and
t.transaction_date < dateadd(month, 1, mon) and
t.customer = c.customer
group by m.mon, c.customer
order by c.ustomer, c.mon ;
Note the other changes to the query:
Year/Month is not a valid column alias.
This orders the rows chronologically. That is usually (always?) preferred over alphabetic sorting of months.

Answer 2

You can use monthYear combination table and with distinct customer list, you can achieve this.
declare #table table(trandate date,customer char(1))
inSert into #table values
('2019-01-03','A'),
('2019-01-17','A'),
('2019-06-03','A'),
('2019-07-03','A'),
('2019-06-03','B'),
('2019-07-03','B');
;with monthYear AS
(
select * from
(
values
('Jan-19')
,('Feb-19')
,('Mar-19')
,('Apr-19')
,('May-19')
,('Jun-19')
,('Jul-19')
,('Aug-19')
,('Sep-19')
,('Oct-19')
,('Nov-19')
,('Dec-19')
) as t(mon)
)
SELECT my.mon,c.customer, isnull(t.salescount,0) as salescount FROM monthYear as my
CROSS JOIN (SELECT distinct customer from #table) as c
OUTER APPLY
(
select format(trandate,'MMM-yy') as MonthYear, count(customer) as salescount
from #table
where customer = c.customer
group by format(trandate,'MMM-yy')
having format(trandate,'MMM-yy') = my.mon
) as t
mon
customer
salescount
Jan-19
A
2
Feb-19
A
0
Mar-19
A
0
Apr-19
A
0
May-19
A
0
Jun-19
A
1
Jul-19
A
1
Aug-19
A
0
Sep-19
A
0
Oct-19
A
0
Nov-19
A
0
Dec-19
A
0
Jan-19
B
0
Feb-19
B
0
Mar-19
B
0
Apr-19
B
0
May-19
B
0
Jun-19
B
1
Jul-19
B
1
Aug-19
B
0
Sep-19
B
0
Oct-19
B
0
Nov-19
B
0
Dec-19
B
0