use tempdb;
go

create table dbo.t1 (id int identity primary key, f int, v int);
create table dbo.t2 (id int identity primary key, f int, v int);

create index IX_t1__f on dbo.t1 (f);
create index IX_t2__f on dbo.t2 (f);
go

set statistics xml on;

update a
 set
  v = b.v
from
 dbo.t1 a join
 dbo.t2 b on b.id = a.id
where
 exists(select a.f intersect select b.f);

update a
 set
  v = b.v
from
 dbo.t1 a join
 dbo.t2 b on b.id = a.id
where
 not exists(select a.f intersect select b.f);

set statistics xml off;
go
 
drop table dbo.t1, dbo.t2;
go