IF ELSE condition with SQL Server trigger - sql

Question

I am populating a table entirely using triggers, it populates the table if LocationID and ProductID does not exists and if it already does it updates the given data.
I have posted the following code snippet looking for a possible solution or link to one.
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER TRIGGER [dbo].[newpurchase]
ON [dbo].[PurchaseMaster]
AFTER INSERT
AS
BEGIN
IF (((SELECT COUNT(*) FROM StockMaster
WHERE LocationID = (SELECT LocationID FROM inserted)) > 0)
AND ((SELECT COUNT(*) FROM StockMaster
WHERE LocationID = (SELECT LocationID FROM inserted)) > 0))
UPDATE StockMaster
SET TotalPurchased = TotalPurchased + (SELECT PurchasedQTY FROM inserted)
WHERE LocationID = (SELECT LocationID FROM inserted)
AND ProductID = (SELECT ProductID FROM inserted);
ELSE
INSERT INTO StockMaster (LocationID, ProductID, TotalPurchased, TotalSold, OnHand)
SELECT LocationID, ProductID, PurchasedQTY, 0, 0 FROM inserted;
END

Answer 1

I am not entirely sure exactly what you are looking for. If your return sets cannot be collected dynamically, and you need another table to always update based off the information persisted by another another table then I do think triggers are fine. There is always overhead with triggers, so keep that in mind.
You could do the way you are doing it, and from what I can see on the surface level the trigger may be what you are looking for (but this also depends upon your business needs and relationships of tables). HOWEVER, you will have to be careful if your Insert is a bulk insert then what you have will throw an error.
I also noticed your If statement is checking LocationId twice instead of LocationId and ProductId.
The trigger you are trying to create works with only one value at a time. You could rewrite this to a more set based trigger for when bulk inserts occur. This way if you have one insert or bulk inserts these two queries can perform for that values where needed.
first you want to update all the values where LocationId and ProductId are the >same found in StockMaster
if no values match then nothing will be updated
Update StockMaster
Set TotalPurchased = sm.TotalPurchased + i.PurchasedQty
From StockMaster sm
inner join inserted i on sm.LocationId = i.LocationId and sm.ProductId = i.ProductId
next you want to insert any row that isn't found in StockMaster
if all values matched above, they will be weeded out with the wheree condition >and,then nothing would be inserted here, those that didn't match would be
inserted
INSERT INTO StockMaster (LocationID, ProductID, TotalPurchased, TotalSold, OnHand)
SELECT i.LocationID, i.ProductID, i.PurchasedQTY, 0, 0
FROM inserted i
left join StockMaster sm on i.LocationId = sm.LocationId and i.ProductId = sm.ProductId
where sm.{Id} is null;--not {id} is for whatever key this table uses