Count Records in Table Except If Related Record Exists in Child Table

Question

Dear All
I have the following query that counts all of the records in the table SENAlert based upon the teacher's username in a related table.
SELECT COUNT(SENAlert.SENAlertID) AS Expr1
FROM Class INNER JOIN ClassMember ON Class.ClassClassCode = ClassMember.ClassMemberClassCode
INNER JOIN Student ON ClassMember.ClassMemberStudentID = Student.StudentID
INNER JOIN SENAlert ON Student.StudentID = SENAlert.SENAlertStudentID
INNER JOIN Teacher ON Class.ClassTeacherCode = Teacher.TeacherCode
WHERE (Teacher.TeacherUsername = 'dsmith')
I need to extend this query by adding another table called SENAlertHistory. I would like to count the number of alerts (SENAlertID) for the specified teacher
but where there is no related record in the SENAlertHistory table. Here's what the relationship diagram looks like:
What's going to happen is when a teacher clicks a button to say they have read an alert, I will record this in the SENAlertHistory table. Therefore when performing a count of how many unread alerts the teacher has, I need to ignore the alerts they have
already read (i.e. the records in the SENAlertHistory table).
This is a bit too advanced for me -- I have tried! I was hoping someone would be able to help me please?
Many thanks
Daniel

Answer 1

Try below code
-- If you dont need the read alert count
SELECT COUNT(SENAlert.SENAlertID) AS Expr1
FROM Class INNER JOIN ClassMember ON Class.ClassClassCode = ClassMember.ClassMemberClassCode
INNER JOIN Student ON ClassMember.ClassMemberStudentID = Student.StudentID
INNER JOIN SENAlert ON Student.StudentID = SENAlert.SENAlertStudentID
INNER JOIN Teacher ON Class.ClassTeacherCode = Teacher.TeacherCode
WHERE (Teacher.TeacherUsername = 'dsmith')
AND NOT EXISTS
(SELECT * FROM SenAlertHistory SAH WHERE Teacher.TeacherCode = SAH.SenAlertHistoryTeacherCode and SENAlert.SENAlertID = SAH.SEMAlertHistorySENAlertID )
-- If you need the read alert count
SELECT COUNT(SENAlert.SENAlertID) - COUNT(SAH.SEMAlertHistorySENAlertID) AS Expr1,COUNT(SAH.SEMAlertHistorySENAlertID)readalert
FROM Class INNER JOIN ClassMember ON Class.ClassClassCode = ClassMember.ClassMemberClassCode
INNER JOIN Student ON ClassMember.ClassMemberStudentID = Student.StudentID
INNER JOIN SENAlert ON Student.StudentID = SENAlert.SENAlertStudentID
INNER JOIN Teacher ON Class.ClassTeacherCode = Teacher.TeacherCode
LEFT OUTER JOIN SenAlertHistory SAH ON Teacher.TeacherCode = SAH.SenAlertHistoryTeacherCode and SENAlert.SENAlertID = SAH.SEMAlertHistorySENAlertID
WHERE (Teacher.TeacherUsername = 'dsmith')
Thanks
Saravana Kumar C