Help With Where Not Exists / Avoiding Loops

I am trying to figure out an efficient way of comparing two tables of identical structure and primary keys only I want to do a join where one of the tables reveals values for records which have been modified and/or updated.

To illustrate, I have two tables in the generic form:

id-dt-val

For which the 'val' in table 2 could be different from the 'val' in table 1 - for a given id-dt coupling that are identical in both tables.

Does anyone know of an efficient way I could return all id-dt couplings in table 2 which have values that are different from those with the same id-dt couplings in table 1?

NOTE: I am asking this because I am trying to avoid explicit comparisons between the 'val' columns. The tables I am working with in actuality have roughly 900 or so columns, so I don't want this kind of a monster query to do (otherwise, I would simply do something like where a.id = b.id and a.dt = b.dt and a.val <> b.val) - but this won't do in this case.

As a sample query, I have the following script below. When I attempt the where not exists, as you might expect, I only get the one record in which the id-dt coupling is different from those in table 1, but I'm not sure how to return the other records where the id-dt coupling is the same in table 1 but for where modified values exist:


create table #tab1
(
id varchar(3),
dt datetime,
val float
)
go

create table #tab2
(
id varchar(3),
dt datetime,
val float
)
go


insert into #tab1
values
('ABC','01/31/1990',5.436)
go
insert into #tab1
values
('DEF','01/31/1990',4.427)
go
insert into #tab1
values
('GHI','01/31/1990',7.724)
go


insert into #tab2
values
('XYZ','01/31/1990',3.333)
go
insert into #tab2
values
('DEF','01/31/1990',11.111)
go
insert into #tab2
values
('GHI','01/31/1990',12.112)
go


select a.* from #tab2 a --Trouble is, this only returns the XYZ record
where not exists
(select b.* from #tab1 b where a.id = b.id and a.dt = b.dt)
go

drop table #tab1
drop table #tab2
go

I really dont' want to have to code up a loop to do the value by value comparison for inequality, so if anyone knows of an efficient set-based way of doing this, I would really appreciate it.

Any advice appreciated!

-KS