Trigger to ensure each new bid is higher than previous - sql

Question

I have a problem with my trigger in SQL Server.
This trigger checks if the new bidding is higher then the existing ones and if not raise an error:
ALTER TRIGGER [dbo].[trg_bod_validate_Bodbedrag]
ON [dbo].[bod]
FOR INSERT, UPDATE
AS
DECLARE #v_Voorwerp numeric(25);
DECLARE #v_Bodbedrag char(6);
DECLARE #v_Max_Bodbedrag char(6);
select #v_Voorwerp = i.voorwerp, #v_Bodbedrag = i.bodbedrag
from Inserted i;
SELECT #v_Max_Bodbedrag = max(CAST(bodbedrag AS INT))
FROM bod
WHERE voorwerp = #v_Voorwerp;
IF #v_Max_Bodbedrag <= #v_Bodbedrag
BEGIN
RAISERROR ('Bod moet hoger zijn dan gegeven bod.', 16, 1)
ROLLBACK TRANSACTION
END;
ELSE
PRINT 'Row Inserted';
Now I get this error Bid amount is less then maximum, that is not acceptable', even when I insert a bidding when there aren't any existing bids.
What could be the problem?
For your knowledge: Voorwerp: Product, Bodbedrag: Amount of bid

Answer 1

Points to note:
In SQL Server the Inserted (and Deleted) pseudo-tables can have from 0-N rows where N is the number of rows being inserted/updated/deleted. And this has to be handled in any trigger. However as soon as you switch into set-based thinking (instead of procedural) you find its a much simpler problem anyway.
Of course we can't (easily) discriminate between rows which are OK and those which aren't, we basically have to reject the entire insert/update even if its just one row which breaks the rules.
So the join finds the max(bodbedrag) for all products which exist in Inserted - excluding the row which is being inserted/updated - because as far as we are concerned the insert/update has already taken place and that data exists in our database - until we rollback if we choose to.
I've ignored your use of a char instead of a decimal. Ideally you would correct your datatypes, but you can continue to cast/convert if you wish. I'll leave that to you.
Note we use throw these days, not raiserror.
ALTER TRIGGER [dbo].[trg_bod_validate_Bodbedrag]
ON [dbo].[bod]
FOR INSERT, UPDATE
AS
BEGIN
SET NOCOUNT ON;
-- Find the max of for inserted voorwerp's excluding the currently inserted/updated
-- Is an update of existing bid even allowed
IF EXISTS (
SELECT 1
FROM Inserted I
LEFT JOIN (
SELECT MAX(bodbedrag) bodbedrag
FROM dbo.dob D
INNER JOIN Inserted I ON I.voorwerp = D.voorwerp and I.ID <> D.ID
) D
WHERE I.bodbedrag < coalesce(D.bodbedrag,0)
) BEGIN
-- THROW should be used now, not RAISERROR
-- RAISERROR ('Bod moet hoger zijn dan gegeven bod.', 16, 1);
THROW 51000, 'Bod moet hoger zijn dan gegeven bod.', 1;
ROLLBACK TRANSACTION;
END; ELSE BEGIN
PRINT 'Row Inserted';
END;
END