Trigger to handle Multi Insert Statements - sql

Question

I have two table Stocks and Purchases
CREATE TABLE Stocks
(
id int PRIMARY KEY IDENTITY(1,1),
itemId int NOT NULL,
qty int NOT NULL,
status NVarChar(50) NOT NULL,
)
CREATE TABLE Purchases
(
id int PRIMARY KEY IDENTITY(1,1),
itemId int NOT NULL,
suppId int NOT NULL,
qty int NOT NULL,
Date NVarChar(50) NOT NULL,
status NVarChar(50) NOT NULL,
)
Here is what I want do:
ON Inserting Multiple Records into Purchases table, I want Create a Trigger that Iterates over each Record Inserted and Check itemId FROM the Stocks Table
UPDATE TheQuantity(qty in this case) of each itemId found AND INSERT into the Stocks Table if itemId isn't found
I have tried several ways, in fact I have been struggling with it since yesterday.
this Post was the closest one I have seen but could figure out what is going on, seems like it's only Updating already existing records
Here is what I have tried so far, I meanthe closest I came
CREATE trigger [dbo].[trPurchaseInsert] on [dbo].[Purchases] FOR
INSERT
AS
DECLARE #id int;
DECLARE #itemId int;
DECLARE #status NVarChar(50) = 'available';
SELECT #itemId=i.ItemId FROM INSERTED i
IF EXISTS(SELECT * FROM Stocks WHERE itemId=#itemId)
BEGIN
SET NOCOUNT ON;
DECLARE #itemIdUpdate int;
DECLARE #qty int;
SELECT #qty=[quantity], #itemIdUpdate=[itemId] FROM INSERTED i
UPDATE Stocks SET qty=qty+#qty WHERE itemId=#itemIdUpdate
END
ELSE
BEGIN
SET NOCOUNT ON;
INSERT INTO Stocks SELECT [itemId], [id], [quantity], #status
FROM INSERTED i
END
this works fine in a single Insert but doesn't work when multiple records are inserted into the Purchase Table at once
The above Trigger Updates the first itemId only and doesn't update the rest or even insert new ones if one itemid is found
The Goal here is to Update in stock items if itemid is found and Insert if itemId isn't found
For Further Details see this SQL Fiddle. It contains Tables & what I have tried with commented details
I have seen several comments advising to use set base operations with joins but couldn't figure a direction
How can I get it to work?

Answer 1

Your trigger is fatally flawed. It does not take into account multiple rows being inserted, it also doesn't deal with updates and deletes.
Instead you should use this:
Note how inserted and deleted are both used. deleted.qty is subtracted.
The whole thing is grouped up by itemId and then the difference is applied to Stocks.
Since you also want to insert, you need to use MERGE. Alternatively, you can have separate joined UPDATE and INSERT...WHERE NOT EXISTS statements.
CREATE OR ALTER TRIGGER dbo.trPurchaseInsert
ON dbo.Purchases
AFTER INSERT, UPDATE, DELETE
AS
SET NOCOUNT ON;
MERGE Stocks s
USING (
SELECT
diff.ItemId,
qty = SUM(diff.qty)
FROM (
SELECT
i.ItemId,
i.qty
FROM inserted i
UNION ALL
SELECT
d.ItemId,
-d.qty
FROM deleted d
) diff
GROUP BY
diff.ItemId
) diff
ON diff.ItemId = s.ItemId
WHEN NOT MATCHED THEN
INSERT (itemId, qty, status)
VALUES (diff.itemId, diff.qty, 'available')
WHEN MATCHED THEN
UPDATE SET
qty += diff.qty;
I must say, I would not implement this with triggers at all.
It's best not to denormalize such data into another table. If you need to query it, you can just do it on the fly from the Purchase table. Use a view if you want, and even index it.
CREATE VIEW dbo.vTotalStock
WITH SCHEMABINDING -- an indexed view must be schema-bound
AS
SELECT
p.ItemId,
qty = SUM(p.qty),
count = COUNT_BIG(*) -- must always include COUNT_BIG if aggregating and indexing
FROM dbo.Purchases p
GROUP BY
p.ItemId;
CREATE UNIQUE CLUSTERED INDEX UX ON vTotalStock (ItemId)
db<>fiddle