Trigger After Update SQL - sql

Question

I have Customer table. To simplify lets say i have two columns
Id
Name
I have a second table (Log) that I want to update ONLY when the Id column of my customer changes. Yes you heard me right that the primary key (Id) will change!
I took a stab but the NewId that gets pulled is the first record in the Customer table not the updated record
ALTER TRIGGER [dbo].[tr_ID_Modified]
ON [dbo].[customer]
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
IF UPDATE (Id)
BEGIN
UPDATE [log]
SET NewId = Id
FROM customer
END
END

Answer 1

Many would make the argument that if you are changing PK values, you need to rethink the database/table design. However, if you need a quick & dirty fix, add a column to the customer table that is unique (and not null). Use this column to join between the [inserted] and [deleted] tables in your update trigger. Here's a sample script:
CREATE TABLE dbo.Customer (
Id INT CONSTRAINT PK_Customer PRIMARY KEY,
Name VARCHAR(128),
UQColumn INT IDENTITY NOT NULL CONSTRAINT UQ_Customer_UQColumn UNIQUE
)
CREATE TABLE dbo.[Log] (
CustomerId INT NOT NULL,
LogMsg VARCHAR(MAX)
)
INSERT INTO dbo.Customer
(Id, Name)
VALUES
(1, 'Larry'),
(2, 'Curley'),
(3, 'Moe')
INSERT INTO dbo.[Log]
(CustomerId, LogMsg)
VALUES
(1, 'Larry is cool'),
(1, 'Larry rocks'),
(2, 'Curley cracks me up'),
(3, 'Moe is mean')
CREATE TRIGGER [dbo].[tr_Customer_Upd]
ON [dbo].[customer]
FOR UPDATE
AS
BEGIN
UPDATE l
SET CustomerId = i.Id
FROM inserted i
JOIN deleted d
ON i.UQColumn = d.UQColumn
JOIN [Log] l
ON l.CustomerId = d.Id
END
SELECT *
FROM dbo.[Log]
UPDATE dbo.Customer
SET Id = 4
WHERE Id = 1
SELECT *
FROM dbo.[Log]