IF statement in SQL Server trigger with condition - database

Question

I created a table student and I want to create a trigger to stop adding
teacher id after 6 to any student so every 5 student have only 1 teacher.
Teacher id in student table is a foreign key so I can repeat in student table
5 times and when I insert 6.. don't agree I try this code but it does not work
The student table looks like this:
CREATE TABLE student
(
s_id int PRIMARY key,
s_name varchar(50) ,
birthday date ,
t_id int
)
And the trigger:
CREATE TRIGGER tr_student
on student
DECLARE
#count int,
#s_id int,
#s_name nvarchar(50),
#birthdate date,
#t_id int
Begin
select #s_id = i.s_id from inserted i;
select #s_name = i.s_name from inserted i;
select #birthdate = i.birthday from inserted i;
select #t_id = i.t_id from inserted i;
Select count(*) t_id
From student
Where #count = t_id
If #count < 6
Insert into student(s_id, s_name, birthday, t_id)
values(#s_id, #s_name, #birthdate, #count)
PRINT 'AFTER INSERT trigger fired'
End

Answer 1

Your fundamental flaw is that you seem to expect the trigger to be fired once per row - this is NOT the case in SQL Server. Instead, the trigger fires once per statement, and the pseudo table Inserted might contain multiple rows.
Given that that table might contain multiple rows - which one do you expect will be selected here??
select #s_id = i.s_id from inserted i;
select #s_name = i.s_name from inserted i;
select #birthdate = i.birthday from inserted i;
select #t_id = i.t_id from inserted i;
It's undefined - you'll get the values from arbitrary rows in Inserted, and you'll ignore all other rows.
You need to rewrite your entire trigger with the knowledge the Inserted WILL contain multiple rows! You need to work with set-based operations - don't expect just a single row in Inserted !
Furthermore: the trigger fires AFTER the insert has already happened, so your code here:
If #count < 6
Insert into student(s_id, s_name, birthday, t_id)
values(#s_id, #s_name, #birthdate, #count)
basically inserts the same data again - certainly not what you want.
So you need to
rewrite your trigger to properly deal with multiple rows in Inserted
either switch to a INSTEAD OF INSERT trigger, or change your trigger logic