SQL Trigger Delete oldest duplicate - sql

Question

I have the following tables
HOLIDAY_DATE_TABLE:
USE BillingUI;
CREATE TABLE HOLIDAY_DATE_TABLE
(
HID INT IDENTITY PRIMARY KEY,
TABLE_NUMBER nchar(2) NOT NULL,
HOLIDAY_DATE nchar(8) NOT NULL,
FIELD_DESCRIPTION nVARchar(43) NULL,
);
tbl8_update_transactions:
USE BillingUI;
CREATE TABLE tbl8_update_transactions
(
TID INT IDENTITY PRIMARY KEY,
TABLE_NUMBER nchar(2) NOT NULL,
HOLIDAY_DATE nchar(8) NOT NULL,
FIELD_DESCRIPTION nVARchar(43) NULL,
HID int,
FOREIGN KEY (HID) REFERENCES HOLIDAY_DATE_TABLE (HID)
);
I'm trying to write a trigger for tbl8_update_transactions. Its goal is to identify records with duplicate foreign key values, and delete all instances of that row except for the most recent (identifiable by having the highest primary key number, as it's auto_incrementing).
What I have so far is....
CREATE TRIGGER tbl8_cleanup
ON tbl8_update_transactions
FOR INSERT
AS
BEGIN
SELECT HID, COUNT(*)
FROM tbl8_update_transactions
GROUP BY HID
HAVING COUNT(*) > 1;
DELETE FROM tbl8_update_transactions
WHERE COUNT(HID) > 1;
END;
I'm unsure as to how I can have the trigger delete all instances of a row that has duplicates except for the instance with the highest primary key number (TID).

Answer 1

I would use row_number() in a CTE:
with todelete as (
select t.*, row_number() over (partition by HID order by TID desc) as seqnum
from tbl8_update_transactions t
)
delete from todelete
where seqnum > 1;

Answer 2

Rather than scanning the entire table every time any activity occurs, why not be more targetted:
CREATE TRIGGER tbl8_cleanup
ON tbl8_update_transactions
INSTEAD OF INSERT --<-- Act before the new rows have been inserted
AS
BEGIN
DELETE FROM tbl8_update_transactions where HID in (select HID from inserted)
INSERT INTO tbl8_update_transactions (/* column list */)
SELECT /* column list */ from inserted
END;
So, before we insert any new rows, we first remove any rows that would be duplicates, and then we perform the actual insert we've been asked to do.
As a general rule, if your trigger code doesn't reference inserted and/or deleted, it's probably broken. See the documentation for more information.