SQL - trigger insert only when a user made an update - sql-server

Question

I have a table, customers_accounts, that tracks some basic information about a customer on an account. When the customer's file is opened, I synchronize the information from an external system so our user gets the most updated information.
UPDATE
customers_accounts
SET
first_name = 'bob',
last_name = 'burger'
WHERE
account_number = '12345'
When a user updates the account, I do the same query, but I update a column indicating the last user to make the change
UPDATE
customers_accounts
SET
first_name = 'bob',
last_name = 'burger',
updated_by = 'H Jon Benjamin',
updated_on = GETDATE()
WHERE
account_number = '12345'
Here's the problem I'm trying to solve. I want to track changes in a history table, but only log changes when they're made by a user, not if they're from the external system. So my plan was to create a trigger that inserts a row if the user column is not blank on the insert (since the updated_by is implicitly null above in the first update)
What I tried is this:
ALTER trigger [dbo].[Accounts_Customers_LogUpdate]
ON [dbo].[Accounts_Customers]
AFTER UPDATE
AS
DECLARE #Now AS DATETIME = GETDATE()
DECLARE #User AS NVARCHAR(150)
SELECT #User = (SELECT [updated_by] FROM INSERTED)
IF (#User IS NOT NULL)
BEGIN
SET NOCOUNT ON
INSERT INTO [dbo].[Accounts_Customers-History]
SELECT *, #User, #Now
FROM inserted
END
Accounts_Customers-History is an exact copy of the table with two additional columns, change_made_by and change_made_on
It doesn't behave how i'd expect though. It inserts whatever value is in updated_by into change_made_by, regardless of the value of updated_by in the query. So I'm getting logged activity triggered by both the user and the import.

Answer 1

Use UPDATE() for this:
Returns a Boolean value that indicates whether an INSERT or UPDATE attempt was made on a specified column of a table or view. UPDATE() is used anywhere inside the body of a Transact-SQL INSERT or UPDATE trigger to test whether the trigger should execute certain actions.
This means that the update function will return false for the first update statement in the question, and true for the second update statement - which is exactly what you need.
Also, please note you should always specify the columns list in an insert statement,
and also always specify the columns list in a select statement. (Why?)
A revised version of your trigger might look something like this:
ALTER TRIGGER [dbo].[Accounts_Customers_LogUpdate]
ON [dbo].[Accounts_Customers]
AFTER UPDATE
AS
DECLARE #Now as DateTime = GetDate()
IF UPDATE(updated_by)
BEGIN
-- Always specify the columns list in an insert statement!
insert into [dbo].[Accounts_Customers-History] (<Columns list>)
-- Always specify the columns list in a select statement!
select <columns list>, #Now
from inserted
END
Please note that the UPDATE() function does not give you any indication if the insert or update statement that fired the trigger was successful, nor does it give you an indication if the value of the column has actually changed - it only indicates whether that column was a part of the insert or update statement that fired the trigger - as you can read in the last paragraph of the remarks section:
If a trigger applies to a column, the UPDATED value will return as true or 1, even if the column value remains unchanged. This is by-design, and the trigger should implement business logic that determines if the insert/update/delete operation is permissible or not.