Why does my trigger to avoid duplicate rows not work? - sql

Question

Before anyone suggests a unique index or key, I have a very good case for this.
I am using this trigger, from Trigger to prevent Insertion for duplicate data of two columns:
CREATE TRIGGER LogDuplicates ON bkPersonPoints
FOR INSERT
AS
if exists (select * from bkPersonPoints c
inner join inserted i
on c.Name = i.Name and c.Points = i.Points)
begin
rollback
end
GO
That answer is accepted and has 15 up-votes, so I would expect it to work, yet even on my very first insert, into an empty table:
insert bkPersonPoints (Name, Points) values ('Brady', 100)
I get the error:
The transaction ended in the trigger. The batch has been aborted.
APPENDIX: The table looks like this:
CREATE TABLE [dbo].[bkPersonPoints](
[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](50) NOT NULL,
[Points] [int] NOT NULL
) ON [APP_BR2_User]

Answer 1

This is happening because it is detecting the record you're currently inserting to the table. You need to filter that out of the EXISTS clause:
CREATE TRIGGER LogDuplicates ON bkPersonPoints
FOR INSERT
AS
if exists (select * from bkPersonPoints c
inner join inserted i
on c.Name = i.Name
and c.Points = i.Points
and c.id <> i.id)
begin
rollback
end
GO

Answer 2

Anyway, suggest CONSTRAINT
ALTER TABLE bkPersonPoints
ADD CONSTRAINT c_uniq_Name_and_points UNIQUE (Name, Points)

Answer 3

This answer was inspired by one posted on Apr 13 '20 at 18:34 in Trigger to prevent Insertion for duplicate data of two columns.
CREATE TRIGGER MyTrigger ON dbo.MyTable
INSTEAD OF INSERT
AS
if not exists (
select * from MyTable t
inner join inserted i
on i.name=t.name and i.date=t.date and i.id <> t.id )
begin
Insert into MyTable (Name, Date) Select Name, Date from inserted
end
else
THROW 51000, 'Statement terminated because a duplicate was found for the object', 1;
go