Difference between COUNT and SUM within an Aggregate CASE statement - sql

Question

Full disclosure, I am learning and I have searched all over the internet and I just can't figure out my question.
I am working on an online class and was given the following example:
select
DATENAME(MONTH, DATEADD(MONTH, MONTH(OrderDate), -1)) AS 'Month',
SUM(CASE WHEN YEAR(OrderDate) = 2005 THEN 1 ELSE 0 END) AS Orders,
SUM(CASE YEAR(OrderDate) WHEN 2005 THEN Totaldue ELSE 0 END) AS 'Total Value'
from
sales.salesorderheader
group by Month(orderdate)
order by Month(orderdate) ASC
That returns the following results:
I understood that (I thought) so I began messing around with the code to further understand Case statements. Looking at the code I thought that the Orders field was essentially finding all the orders in a month, assigning a 1 to each one, and then adding them all up. Because each one was assigned a 1 I figured that I could change the SUM to COUNT and I would get the same results.
However, this code:
select
DATENAME(MONTH, DATEADD(MONTH, MONTH(OrderDate), -1)) AS 'Month',
COUNT(CASE WHEN YEAR(OrderDate) = 2005 THEN 1 ELSE 0 END) AS Orders,
SUM(CASE YEAR(OrderDate) WHEN 2005 THEN Totaldue ELSE 0 END) AS 'Total Value'
from
sales.salesorderheader
group by Month(orderdate)
order by Month(orderdate) ASC
Returns these results:
To try and break this down I created a query that would just look for the orders in January 2005 and count them.
SELECT COUNT(*)
FROM Sales.SalesOrderHeader
WHERE OrderDate >= '1/1/2005' AND OrderDate < '1/1/2005'
This returned 0. The same as the SUM query. I get that COUNT counts rows and SUM sums numbers in a column, but I just don't understand the results I'm getting. Could someone please explain why the count query is returning 2483 for January and not 0?

Answer 1

For COUNT 1 and 0 are the same. What you really need is NULL:
COUNT(ALL expression) evaluates expression for each row in a group and returns the number of nonnull values.
select
DATENAME(MONTH, DATEADD(MONTH, MONTH(OrderDate), -1)) AS 'Month',
COUNT(CASE WHEN YEAR(OrderDate) = 2005 THEN 1 ELSE NULL END) AS Orders,
SUM(CASE YEAR(OrderDate) WHEN 2005 THEN Totaldue ELSE 0 END) AS 'Total Value'
from sales.salesorderheader
group by Month(orderdate)
order by Month(orderdate) ASC;
Or even shorter(default ELSE is NULL so we could omit that part)
COUNT(CASE WHEN YEAR(OrderDate) = 2005 THEN 1 END) AS Orders,
Example:
SUM COUNT COUNT
2005 1 1 1
2006 0 0 NULL
2007 0 0 NULL
2005 1 1 1
===============================================
2 4 2

Answer 2

When you use count(*) you count ALL the rows. If you want to count how many orders you have, you have to use a column: eg: count(OrderDate). Try it

Answer 3

count example:
assume that your column has 3 value and column name is the order
2 ---------- 5 ---------- 4----- Null
now if you run
count (order)
it will return = 3 how many entries you have in the column without null
sum example:
2 ---------- 5 ---------- 4
now if you run
sum (order)
it will return = 2+5+4=11 its add all the entries