Can I shorten this SQL query? - sql

Question

I have write a query to get the total number assignments which had no activity from last 5 months including those assignments which were created five months ago and never had any visit on it.
Can i shorten this query any further so i don't have to use the outer query.
SELECT #NumNoActivity = COUNT(QnoActivity.AssignmentID) FROM
(
SELECT a.AssignmentID
FROM Assignments a
LEFT JOIN VISITS v ON v.AssignmentID = a.AssignmentID
WHERE a.CurrentStatus = 1
AND a.StaffID = #StaffID
GROUP BY a.AssignmentID, a.CreatedDate
HAVING DATEDIFF(MONTH, ISNULL(MAX(v.VisitDate),a.CreatedDate ), GETDATE()) > =5
) QnoActivity

Answer 1

You could use NOT EXISTS to remove the join:
SELECT #NumNoActivity = COUNT(a.AssignmentID)
FROM Assignments a
WHERE a.CurrentStatus = 1
AND a.StaffID = #StaffID
AND NOT EXISTS
(SELECT * FROM VISITS v
WHERE v.AssignmentID = a.AssignmentID
AND v.VisitDate>DATEADD(month,-5,GETDATE()))
UPDATE
Based on Gordon Linoff's comment, the query was missing the Assignments.CreatedDate condition. Here's an updated version:
SELECT #NumNoActivity = COUNT(a.AssignmentID)
FROM Assignments a
WHERE a.CurrentStatus = 1
AND a.StaffID = #StaffID
AND (SELECT ISNULL(MAX(VisitDate), a.CreatedDate) FROM VISITS v
WHERE v.AssignmentID = a.AssignmentID) <= DATEADD(month,-5,GETDATE())
Here is SQL Fiddle to show how it works. Assignment results are:
1 - not included as there's a recent visit
2 - included as there's no visit and create date is old
3 - included as there's only a recent visit
4 - not included as there's no visit but create date is recent
5 - not included as there's a recent visit

Answer 2

There is an alternative way to write this query. You can look at it as the difference etween the total number of "assignments" and the total number of "assignments" that have had activity in the past 4/5 months.
The following takes this approach. It excludes new "assignments" in the where clause and the counts the different in the select clause:
SELECT (count(distinct a.AssignmentID) -
count(distinct case when datediff(MONTH, v.VisitDate, GETDATE()) < 5 or
v.VisitDate is null
then a.AssignmentId end)
)
FROM Assignments a LEFT JOIN
VISITS v
ON v.AssignmentID = a.AssignmentID
WHERE a.CurrentStatus = 1 AND
a.StaffID = #StaffID and
a.CreatedDate <= DATEADD(month, -5, GETDATE());