MSSQL MERGE runs update triggers

gistfile1.sql

-- This behavior is correct, just surprising if you havn't designed your trigger correctly.
-- Standard cause is designing a trigger that expects exactly 1 row and only 1 row to be changed
-- and doesn't account for if no rows are updated, or more than 1 row is updated.

create table foo (id int primary key, value nvarchar(255));
go

create trigger t_foo_update on foo for update as
set nocount on

select
  (select count(*) from inserted) as inserted_count,
  (select count(*) from deleted) as deleted_count
go

print 'inserting'
insert into foo (id, value) values (1, 'test');
GO

print 'updating'
update foo set value = 'bob' where id = 1;
GO

print 'merging - insert new record, note that update is still fired with 0 rows'
merge foo as target
using (select 2, 'jim') as source (id, value)
on target.id = source.id
when matched then update set value = source.value
when not matched then insert (id, value) values (source.id, source.value);
GO