Update DateUsed by trigger - sql-server

Question

I'm trying to update a table with a trigger from another table. I thought this would be a very simple query but the query I first came up with does not work and I don't understand why.
CREATE TABLE [dbo].[Vehicle](
[id] [int] IDENTITY(1,1) NOT NULL,
[plate] [nvarchar](50) NOT NULL,
[name] [nvarchar](50) NOT NULL,
[dateUsed] [datetime] NULL
)
CREATE TABLE [dbo].[Transaction](
[id] [int] IDENTITY(1,1) NOT NULL,
[vehicleId] [int] NOT NULL,
[quantity] [float] NOT NULL,
[dateTransaction] [datetime] NOT NULL,
)
When a transaction is added, I wish to update the Vehicle table. If the added dateTransaction is later then dateUsed it should be updated so the dateUsed field always contains the latest date of that specific vehicle.
I would think that this trigger should do the trick.. but it does not:
UPDATE [Vehicle]
SET [dateUsed] =
CASE
WHEN [dateUsed] < [Transaction].[dateTransaction]
OR [dateUsed] IS NULL
THEN [Transaction].[dateTransaction]
ELSE [dateUsed]
END
FROM [Transaction]
WHERE [Vehicle].[id]=[Transaction].[vehicleId]
It looks good to me... It should go over all newly inserted records and update the dateUsed field. If the dateTransaction is newer, use that one.. if not.. use the current. But I seem to missing something because it's not updating to the latest date. It does match one of the transactions of that specific vehicle but not the latest one.
A query that does work:
UPDATE [Vehicle]
SET [dateUsed] = InsertedPartitioned.[dateTransaction]
FROM [Vehicle]
LEFT JOIN (
SELECT
[vehicleId],
[dateTransaction],
ROW_NUMBER() OVER(PARTITION BY [VehicleId] ORDER BY [dateTransaction] DESC) AS RC
FROM [Inserted]) AS InsertedPartitioned
ON InsertedPartitioned.RC=1
AND InsertedPartitioned.[vehicleId]=[Vehicle].[id]
WHERE InsertedPartitioned.[vehicleId] IS NOT NULL
AND ([Vehicle].[dateUsed] IS NULL
OR InsertedPartitioned.[dateTransaction] > [Vehicle].[dateUsed]);
So I have a working solution and it may even be for the better (haven't timed it with a large insert) but it bugs the hell out of my not knowing why the first it not working!
Can anyone 'enlighten me'?

Answer 1

why the first it not working
Because of a wonderful aspect of the Microsoft extension to UPDATE that uses a FROM clause:
Use caution when specifying the FROM clause to provide the criteria for the update operation. The results of an UPDATE statement are undefined if the statement includes a FROM clause that is not specified in such a way that only one value is available for each column occurrence that is updated, that is if the UPDATE statement is not deterministic.
(My emphasis).
That is, if more than one row from inserted matches the same row in Vehicle then it's undefined which row will be used to apply the update - and all computations within the SET clause are computed "as if" they're all evaluated in parallel - so it's not as if a second attempt to update the same row will observe the results on the first attempt - the current value of the DateUsed column that can be observed is always the original value.
In ANSI standard SQL, you'd have to write the UPDATE without using the FROM extension and would thus have to write a correlated subquery, something like:
UPDATE [Vehicle]
SET [dateUsed] = COALESCE((SELECT dateUsed FROM inserted i
WHERE i.VehicleId = Vehicle.Id and
(i.dateUsed > Vehicle.DateUsed or
Vehicle.DateUsed IS NULL),
dateUsed)
WHERE [id] IN (select [vehicleId] FROM inserted)
Which, under the same circumstances, would nicely give you an error about a subquery returning more than one value (for the one inside the COALESCE, not the one in the IN) and thus give you a clue to why it's not working.
But, undeniably, the FROM extension is useful - I just wish it triggered a warning for this kind of situation.