Question

I’ve discovered a new problem: I have a two tables, Classes and Students. Students references to Classes by [ClassID] column. Classes have column named [Count], which storing count of referencing students and I’m trying update it via AFTER INSERT,DELETE trigger on Students table.
I wrote a simple CALC_COUNT procedure like that:

---

CREATE PROCEDURE [dbo].[CALC_COUNT]
    @classid INT
AS
BEGIN
UPDATE classes SET [Count] = (SELECT COUNT(Id) FROM students WHERE [ClassID] = @classid);
END

RETURN 0

---

and use it inside trigger

---

CREATE TRIGGER [MONITOR_STUDENTS_SCHEMA_TRIGGER]
    ON [dbo].[students]
    AFTER DELETE, INSERT
    AS
    BEGIN
        UPDATE [dbo].[classes] 
        SET studentsschemarev +=1  FROM inserted; 
        CALC_COUNT(SELECT [ClassID] FROM inserted UNION SELECT [ClassID] FROM deleted); 
        UPDATE [dbo].[stats] SET students_schema_rev += 1;
    END

---

But it not works.
I think, I need a way to execute procedure for each row in SELECT statement of trigger, but I don’t know how.

SQL Server 2012 LocalDB, compatibility mode with SQL Server 2008.

Answer 1

You don’t need your stored proc.

Update your trigger to be

update classes
set 
   count = StudentCount,
   schemarevcount += 1
from
    classes 
        inner join
    (select * from inserted union select * from deleted) students
        on classes.classid=students.classid
        inner join 
    (select classid, count(*) as StudentCount from students group by classid) counts
        on students.classid = counts.classid

in place of the update and the call to calc_count

An english translation…

• Update classes (set the revision and the count)
• where the class is changed in the students table
• where that class is in the set of counts of students per class