Count the number of rows each month in SQL Server - sql

Question

I'm using SQL Server. I've a following table Orders:
Orders (Id, ItemId, CustomerId, Quantity, OrderDateTime)
I want to count the number of orders each month. I've written 2 of the following query.
Query #1:
SELECT
MONTH(OrderDateTime) AS MonthCol,
YEAR(OrderDateTime) AS YearCol,
COUNT(id) AS OrderCount
FROM
Orders
WHERE
OrderDateTime >= '2000' AND OrderDateTime <= '2018'
GROUP BY
YEAR(OrderDateTime), MONTH(OrderDateTime)
ORDER BY
YearCol, MonthCol
Query #2:
SELECT
DATEPART(mm, OrderDateTime) AS Month,
COUNT(*) AS OrderCount
FROM
Orders
WHERE
OrderDateTime >= '2000' AND OrderDateTime <= '2018'
GROUP BY
DATEPART(mm, OrderDateTime)
Issue with both queries is that I'm not getting the columns with 0 orders. How will I get it?

Answer 1

SQL will not give you data about months and year which do not exist as rows. To get 0 order rows you'd need to right join the results with a calendar table containing all needed months and years or you can also use a tally table.
Select T.MonthCol, T.YearCol,OrderCount= COALESCE(OrderCount,0)
from
(
SELECT MONTH(OrderDateTime) AS MonthCol, YEAR(OrderDateTime) AS YearCol, count(id) AS OrderCount
FROM Orders
WHERE OrderDateTime >= '2000' AND OrderDateTime <= '2018'
GROUP BY YEAR(OrderDateTime), MONTH(OrderDateTime)
ORDER BY YearCol, MonthCol)
P
RIGHT JOIN
(
select * from
( values (2000),(2001),(2002),(2003),(2004),(2005),(2006),(2007),(2008))v(YearCol)
cross join
( values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12))u(MonthCol)
)T
on P.MonthCol=T.MonthCol
and P.YearCol=T.YearCol

Answer 2

I would be inclined to use a recursive CTE for this -- this gives pretty easy flexibility on the range you want:
with dates as (
select cast('2000-01-01' as date) dte
union all
select dateadd(month, 1, dte)
from dates
where dte < '2018-12-01'
)
select year(OrderDateTime) AS year,
month(OrderDateTime) AS month,
count(o.id) as OrderCount
from dates left join
orders o
on d.OrderDateTime >= dates.dte and
d.OrderDateTime < dateadd(month, 1, dates.dte)
group by year(OrderDateTime), month(OrderDateTime)
order by year(OrderDateTime), month(OrderDateTime)
option (maxrecursion 0);
Notes:
This uses the JOIN to do the filtering. This makes it safer to change the range that you are looking for.
I find the year() and month() functions to be more convenient datepart().
When using date parts, spell them out. Why waste brain power trying to remember if mm really means months or minutes?
I added an order by. Presumably you want the results in chronological order.