converting a mySQL Trigger, to TSQL - mysql

Question

Looking for help converting this mySQL Trigger, to TSQL.
CREATE TRIGGER IF not exists TF_WATCHLIST_PERSISTENCE_ADD_POSITION
AFTER INSERT ON TF_WATCHLIST_PERSISTENCE
FOR EACH ROW
WHEN NEW.WATCHLIST_POSITION IS NULL
BEGIN
UPDATE TF_WATCHLIST_PERSISTENCE SET
WATCHLIST_POSITION = (SELECT COUNT(*) - 1 FROM TF_WATCHLIST_PERSISTENCE WHERE WATCHLIST_USER = NEW.WATCHLIST_USER)
WHERE rowid = NEW.rowid;
END;

Answer 1

Thanks for all your help
I ended up with this.
CREATE TRIGGER TF_WATCHLIST_PERSISTENCE_ADD_POSITION
ON TF_WATCHLIST_PERSISTENCE
AFTER INSERT, UPDATE
AS
DECLARE #MyCursor CURSOR;
DECLARE #MyField_USER varchar(250);
DECLARE #MyField_ID varchar(250);
DECLARE #MyField_NewPos int
BEGIN
SET #MyCursor = CURSOR FOR
SELECT WATCHLIST_USER
, WATCHLIST_ID
FROM inserted
WHERE inserted.WATCHLIST_POSITION is null
OPEN #MyCursor
FETCH NEXT FROM #MyCursor
INTO #MyField_USER, #MyField_ID
WHILE ##FETCH_STATUS = 0
BEGIN
set #MyField_NewPos = (SELECT MAX(WATCHLIST_POSITION) + 1 FROM TF_WATCHLIST_PERSISTENCE WHERE WATCHLIST_USER = #MyField_USER)
print #MyField_NewPos
update TF_WATCHLIST_PERSISTENCE
SET WATCHLIST_POSITION = ISNULL(#MyField_NewPos,0)
WHERE WATCHLIST_USER = #MyField_USER
AND WATCHLIST_ID = #MyField_ID
AND WATCHLIST_POSITION IS NULL
Print #MyField_USER
Print #MyField_ID
FETCH NEXT FROM #MyCursor
INTO #MyField_USER, #MyField_ID
END;
END;
CLOSE #MyCursor

Answer 2

Try this:
CREATE TRIGGER TF_WATCHLIST_PERSISTENCE_ADD_POSITION ON TF_WATCHLIST_PERSISTENCE AFTER INSERT
AS
BEGIN
WITH DataSource AS
(
SELECT DS.WATCHLIST_USER
,COUNT(*) - 1 AS WATCHLIST_POSITION
FROM TF_WATCHLIST_PERSISTENCE DS
INNER JOIN inserted I
ON DS.WATCHLIST_USER = I.WATCHLIST_USER
WHERE inserted.WATCHLIST_POSITION is null
GROUP BY DS.WATCHLIST_USER
)
UPDATE TF_WATCHLIST_PERSISTENCE
SET WATCHLIST_POSITION = DS.[WATCHLIST_POSITION]
FROM TF_WATCHLIST_PERSISTENCE DS
INNER JOIN DataSource I
ON DS.WATCHLIST_USER = I.WATCHLIST_USER;
END

Answer 3

I suspect you are looking for a ranking function, or even a simple COUNT(*) per user? MySQL is rather poor in analytic, ranking or windowing functions so tricks like quirky updates or triggers are used to emulate them.
In SQL Server, you could return the number of records that match a single user in each row with COUNT() applied over the records that match a single user :
SELECT *, COUNT(*) OVER (PARTITION BY WATCHLIST_USER)
FROM TF_WATCHLIST_PERSISTENCE
No need for a trigger. You can use any aggregate with an OVER() clause.
If you want the rank according to some order, you can use one of the ranking functions like ROW_NUMBER() or `RANK(). If you want to calculate a row position per user based, eg on creation date you could write :
SELECT *, ROW_NUMBER(*) OVER (PARTITION BY WATCHLIST_USER ORDER BY CreationDate)
FROM TF_WATCHLIST_PERSISTENCE
If you have an incrementing ID you could use it to generate a row position per user
SELECT *, ROW_NUMBER(*) OVER (PARTITION BY WATCHLIST_USER ORDER BY ID)
FROM TF_WATCHLIST_PERSISTENCE
Most of the time, you don't need to store the row position. It's simple enough to add a ranking function to a query.
If you do need to store such a row position in a table, you could use a simple multi-row trigger
CREATE TRIGGER TF_WATCHLIST_PERSISTENCE_ADD_POSITION
ON TF_WATCHLIST_PERSISTENCE
AFTER INSERT, UPDATE
AS
update TF_WATCHLIST_PERSISTENCE
set WATCHLIST_POSITION =ROW_NUMBER() OVER (PARTITION BY WATCHLIST_USER ORDER BY ID)
FROM inserted
INNER JOIN TF_WATCHLIST_PERSISTENCE on inserted.ID=TF_WATCHLIST_PERSISTENCE.ID
Or
update TF_WATCHLIST_PERSISTENCE
set WATCHLIST_POSITION =ROW_NUMBER() OVER (PARTITION BY WATCHLIST_USER ORDER BY ID)
FROM inserted
INNER JOIN TF_WATCHLIST_PERSISTENCE
ON inserted.WATCHLIST_USER =TF_WATCHLIST_PERSISTENCE.WATCHLIST_USER
If you want to insert new entries for an existing user and recalculate all positions