Trigger for UPDATE runs many time on batch Updates - sql

Question

All of my tables have a Trigger for CRUD operations.
here is a sample:
ALTER TRIGGER [dbo].[Cities_tr] ON [dbo].[Cities] AFTER INSERT, UPDATE
AS
BEGIN
DECLARE #operation CHAR(6)
SET #operation = CASE WHEN EXISTS (SELECT * FROM inserted) AND EXISTS (SELECT * FROM deleted)
THEN 'Update'
WHEN EXISTS (SELECT * FROM inserted)
THEN 'Insert'
WHEN EXISTS(SELECT * FROM deleted)
THEN 'Delete'
ELSE NULL
END
IF #operation = 'Insert'
INSERT INTO history ([dt],[tname],[cuser] ,[id],op)
SELECT GETDATE(),'Cities', i.ldu, i.CityId,#operation
FROM inserted i
set nocount on
IF #operation = 'Update'
INSERT INTO history ([dt],[tname],[cuser] ,[id],op)
SELECT GETDATE(),'Cities', i.ldu, i.CityId,#operation
FROM deleted d, inserted i
END
If I update one row, everything works fine and trigger inserts one row in history.
For example
update top(1) cities set f=1
But if more than one row updated, updatedrow^2 rows will be inserted.
For example 9 for 3 rows 100 for 10 rows...
What is wrong with my trigger and how could I solve it?

Answer 1

The problem with your code is that you are cross joining inserted and deleted. On a multi-rows update, both contain many rows, which the cartesian product multiplies.
It looks like you actually want to log the "new" rows (either inserted or updated). If so, you don’t want to select from deleted. Also, the conditional logic can be moved within a single query, which allows to simplify your code as follows:
ALTER TRIGGER dbo.Cities_tr
ON dbo.Cities
AFTER INSERT, UPDATE
AS
BEGIN
INSERT INTO history (dt, tname, cuser, id, op)
SELECT
getdate(),
'Cities',
ldu,
cityId,
case when exists (select 1 from deleted) then 'Update' else 'Insert' end
FROM inserted;
END
On the other hand, if you want to log both the "old" and "new" rows (which is not what your code does, even on a single-row update), then you want to union all two queries that select from inserted and deleted.

Answer 2

You are cross joining inserted and deleted. Normally, they would be joined using the table's primary key, which is presumably CityId:
INSERT INTO history ([dt], [tname], [cuser] , [id], op)
SELECT GETDATE(), 'Cities', i.ldu, i.CityId, #operation
FROM deleted d JOIN
inserted i
ON d.CityId = i.CityId;
In this case, deleted is not being used, so it does not even need to be included in the query.
You could implement the entire trigger as a single query in the table using LEFT JOIN:
INSERT INTO history ([dt], [tname], [cuser] , [id], op)
SELECT GETDATE(), 'Cities', i.ldu, i.CityId,
(CASE WHEN d.CityId IS NOT NULL THEN 'Update' ELSE 'Insert' END)
FROM inserted i LEFT JOIN
deleted d
ON d.CityId = i.CityId;