Microsoft sql trigger update multiple lines - sql-server

Question

I have a two tables, a Customers and a Sales table.
I am trying to create a trigger to update the amount of sales in the customer table when the Sales table is updated.
CREATE TRIGGER salesUPDATE
ON SALES
AFTER INSERT
AS
UPDATE Customers
SET salesAmount = Sales.Amount
GO
But I get that sales does not exist. Should I be using a join?
Will this trigger update all columns or do I need to specify which column to update?

Answer 1

CREATE TRIGGER salesUpdate ON SALES
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
;WITH cteAffectedCustomers AS (
SELECT DISTINCT CustomerId
FROM
inserted
UNION
SELECT DISTINCT CustomerId
FROM
deleted
)
, cteAggregations AS (
SELECT
ca.CustomerId
,SUM(ISNULL(s.Amount,0)) as SalesAmount
,COUNT(s.SalesId) as NumOfSales
FROM
cteAffectedCustomers ca
INNER JOIN Customers c
ON ca.CustomerId = c.CustomerId
LEFT JOIN Sales s
ON ca.CustomerId = s.CustomerId
GROUP BY
ca.CustomerId
)
UPDATE c
SET SalesAmount = ca.SalesAmount
,NumOfSales = ca.NumOfSales
FROM
Customers c
INNER JOIN cteAggregations ca
ON c.CustomerId = ca.CustomerId
END
Here is an example of the type of logic you would need to create to maintain a pre-agregated value. If you want to SUM an Amount in the Sales table you will need an AFTER INSERT, UPDATE, and DELETE. Then you would need to:
determine all of the affected customers So you don't update the entire customer table
do the aggregation
update with an inner join to the aggregated data
A note about triggers, they are a set based operation NOT a scalar. That means they fire once for x# of rows NOT x# of times for x# of rows. So you have to account for multiple records during updates and do joins just like would outside of a trigger when updating one table with another.
This has a performance impact to write operations but does expedite your reads, however if you are not in an extremely extremely high read volume operation you would do better to use a view/query and optimize your indexes. There is less likely hood of the synchronization of the aggregate data getting messed up. If you do go the trigger route I suggest you also have a SQL job set up on some reasonable increment (nightly) that checks and rectifies any inconsistencies that may occur.

Answer 2

Use magic table inserted
CREATE TRIGGER salesUPDATE
ON SALES
AFTER INSERT
AS
BEGIN
Declare #Amount varchar(50) = (Select top 1 Amount from inserted)
UPDATE Customers
SET salesAmount = #Amount
END
GO
Note : top 1 for insertion of multiple records

Answer 3

Use the inserted table name which contains the new value:
CREATE TRIGGER salesUPDATE
ON SALES
AFTER INSERT
AS
UPDATE Customers
SET salesAmount = inserted.Amount
GO