How Do I Write A Sql Trigger In Sql Server 2008

create table mytable(
	id int IDENTITY(1,1) PRIMARY KEY,
	columnA nvarchar(5) ,
	columnB nvarchar(5)
	)
GO
CREATE TRIGGER [dbo].[myname]
ON [dbo].[mytable] 
FOR UPDATE
AS
BEGIN
	declare @targets table(
		id int
	)
	insert into @targets(id)
	(select id from deleted)

	declare @id int

	set @id = (select top 1 id from @targets)
	while exists(select * from @targets)
	BEGIN
		declare @a nvarchar(5)
		declare @b nvarchar(5)
		set @a = (select columnA from deleted where id = @id)
		set @b = (select columnB from deleted where id = @id)

		if (Update(columnA) or Update(columnB)) and @a <> @b
		begin
			update mytable
			set columna = columnb
			where id = @id
		end

		delete from @targets where id = @id;
		set @id = (select top 1 id from @targets)
	END
END
go

insert into mytable(ColumnA, ColumnB)
values('abc','bcd')

update mytable
set columna = 'gef'
where id = 1

select * from mytable

drop table mytable