IF statements in SQL Server triggers - if-statement

Question

I need to create a SQL Server trigger to block updates and deletes to a table Service.
This action should be done only to Service in which the column States sample data is "completed".
It should allow updates and deletes to Service in which the column States sample data is "active".
This is what I tried, I am having problems with doing the else operation (that is allowing updates to Service in which the column State sample data is "active").
CREATE TRIGGER [Triggername]
ON dbo.Service
FOR INSERT, UPDATE, DELETE
AS
DECLARE #para varchar(10),
#results varchar(50)
SELECT #para = Status
FROM Service
IF (#para = 'completed')
BEGIN
SET #results = 'An invoiced service cannot be updated or deleted!';
SELECT #results;
END
BEGIN
RAISERROR ('An invoiced service cannot be updated or deleted', 16, 1)
ROLLBACK TRANSACTION
RETURN
END

Answer 1

So if I understand you correctly, any UPDATE or DELETE should be allowed if the State column has a value of Active, but stopped in any other case??
Then I'd do this:
CREATE TRIGGER [Triggername]
ON dbo.Service
FOR UPDATE, DELETE
AS
BEGIN
-- if any row exists in the "Deleted" pseudo table of rows that WERE
-- in fact updated or deleted, that has a state that is *not* "Active"
-- then abort the operation
IF EXISTS (SELECT * FROM Deleted WHERE State <> 'Active')
ROLLBACK TRANSACTION
-- otherwise let the operation finish
END
As a note: you cannot easily return messages from a trigger (with SELECT #Results) - the trigger just silently fails by rolling back the currently active transaction