How can I update a column in SQL Server using a trigger - sql-server

Question

I am trying to create a trigger within SQL Server Management Studio that will increment a column value by 1 when a separate column has been updated within the same table.
The value for the column we want to update when the update script has been ran becomes NULL
My example is that I when I change the address of a customer, I want a column that goes up by 1 every time the address is changed i.e NoOfAddressess = 1, 2, 3 etc...
Here is the SQL code that I am writing
ALTER TRIGGER trg_customeraudit
ON tblCustomer
AFTER UPDATE, DELETE, INSERT
AS
INSERT INTO dbo.CustomerDetailsAudit
VALUES (CURRENT_USER, CURRENT_TIMESTAMP,
(SELECT CustomerID FROM inserted),
(SELECT CustomerAddress FROM deleted),
(SELECT CustomerAddress FROM inserted),
(SELECT CustomerPostcode FROM deleted),
(SELECT CustomerPostcode FROM inserted),
(SELECT NumberOfChangedAddresses FROM dbo.CustomerDetailsAudit)
)
IF ((SELECT CustomerAddress FROM inserted) =
(SELECT CustomerAddress FROM deleted) OR
(SELECT CustomerPostcode FROM deleted) =
(SELECT CustomerPostcode FROM inserted))
BEGIN
RAISERROR ('You must enter both a new postcode and address',16,10)
ROLLBACK TRANSACTION
END
ELSE
BEGIN
PRINT 'Transaction successful'
WHERE CustomerID = (SELECT CustomerID from inserted)
END
IF UPDATE (CustomerName)
BEGIN
RAISERROR ('You cannot change the customer name', 16, 10)
ROLLBACK
END

Answer 1

Depending on the other things happening on this data triggers can be a very inefficient method of handling this, but here is one possible solution.
1. Setup
First create a table to use for testing.
create table test_table (
MyPrimaryKey int primary key clustered not null identity(1, 1)
, SomeColumn varchar(255) not null
, SomeColumnCounter int null
);
go
Now, add a trigger to initialize the counter to 1. This could be handled by a default constraint or set at the application level but it can also be done with a trigger.
-- this trigger will set the counter to 1 when a record is first added
-- doesn't need to be a trigger, but since the question was on triggers
create trigger trg_test_table_insert
on test_table
after insert
as
update tt
set tt.SomeColumnCounter = 1
from
test_table as tt
inner join
Inserted as i
on
tt.MyPrimaryKey = i.MyPrimaryKey;
go
Now, add a trigger that will check for changes on the designated column and increment the counter if needed.
-- this trigger will increment the counter by 1 if 'SomeColumn' changed
-- doesn't handle nulls so will need to be modified depending on schema
create trigger trg_test_table_update
on test_table
after update
as
update tt
set tt.SomeColumnCounter = tt.SomeColumnCounter + 1
from
Inserted as i -- new version of the record
inner join
Deleted as d -- old version of the record
on
i.MyPrimaryKey = d.MyPrimaryKey
and i.SomeColumn <> d.SomeColumn
inner join
test_table as tt
on
tt.MyPrimaryKey = i.MyPrimaryKey
go
2. Testing
Add some test data.
insert into test_table (SomeColumn)
values ('abc'), ('def');
go
Now we have:
MyPrimaryKey SomeColumn SomeColumnCounter
1 abc 1
2 def 1
Update without changing anything:
update tt
set tt.SomeColumn = 'abc'
from
test_table as tt
where
tt.MyPrimaryKey = 1
We still have:
MyPrimaryKey SomeColumn SomeColumnCounter
1 abc 1
2 def 1
Update that actually changes something:
update tt
set tt.SomeColumn = 'abbc'
from
test_table as tt
where
tt.MyPrimaryKey = 1
Now we have:
MyPrimaryKey SomeColumn SomeColumnCounter
1 abbc 2
2 def 1
Update that changes everything:
update tt
set tt.SomeColumn = tt.SomeColumn + 'z'
from
test_table as tt
Now we have:
MyPrimaryKey SomeColumn SomeColumnCounter
1 abbcz 3
2 defz 2