Trigger to update sum of columns? - sql

Question

I know this is not a way to do it but it's a interview question
to update total = marks1 + marks2 + marks3 using a trigger.
I wrote something like this but it's not updating after an insert statement.
CREATE table marks
(
marks1 int,
marks2 int,
marks3 int,
total int
)
SELECT * from marks m
insert into marks values(10,10,20,0)
drop TRIGGER total_marks
create TRIGGER total_marks ON marks
AFTER INSERT
AS
begin
SET NOCOUNT ON
DECLARE #marks1 as int
select #marks1 = inserted.marks1 FROM inserted
DECLARE #marks2 as int
select #marks1 = inserted.marks2 FROM inserted
DECLARE #marks3 as int
select #marks1 = inserted.marks3 FROM inserted
DECLARE #result as int
set #result = #marks1 + #marks2 + #marks3
update marks
set total = #result
SET NOCOUNT OFF
end

Answer 1

Your trigger doesn't handle multiple row inserts, updates all rows to the same value (rather than just the row(s) inserted), and is far more complex than necessary anyway. Where is your key?
CREATE TRIGGER dbo.total_marks
ON dbo.marks
FOR INSERT
AS
BEGIN
SET NOCOUNT ON;
UPDATE m
SET total = i.marks1 + i.marks2 + i.marks3
FROM dbo.marks AS m
INNER JOIN inserted AS i
ON m.key = i.key;
END
GO
If your table really doesn't have a key (it doesn't make a whole lot of sense to me), then you can say this, but it may update rows that were already updated:
ON m.marks1 = i.marks1 AND m.marks2 = i.marks2 AND m.marks3 = i.marks3
WHERE m.total = 0;