-- preparing workspace --
 
create table test1 (f1 int null,f2 char( 3 ) null,f3 int null)
go
create table test2 (f11 int null,f12 char( 3 ) null,f13 int null)
go
insert test1 (f1,f2,f3) values( 1 ,'А', 10 )
insert test1 (f1,f2,f3) values( 2 ,null, 20 )
insert test1 (f1,f2,f3) values( 3 ,'В', 20 )
go
insert test2 (f11,f12,f13) values( 1 ,'А', 10 )
insert test2 (f11,f12,f13) values( 2 ,'Б', 20 )
insert test2 (f11,f12,f13) values( 3 ,'В', 20 )
insert test2 (f11,f12,f13) values( 4 ,'Г', 20 )
insert test2 (f11,f12,f13) values( 5 ,'Д', 30 )
go

 -- selecting data --
 
select * from test1 t1, test2 t2
where (t1.f2 = t2.f12 and t1.f3 = t2.f13)
    or ( coalesce(t1.f2,'NULL') = 'NULL' and t1.f3 = t2.f13)
go

 -- get results --
 
f1          f2   f3          f11         f12  f13         
 ----------- ---- ----------- ----------- ---- ----------- 
 
 1 . 00         А     10 . 00         1 . 00         А     10 . 00 
 2 . 00         NULL  20 . 00         2 . 00         Б     20 . 00 
 2 . 00         NULL  20 . 00         3 . 00         В     20 . 00 
 2 . 00         NULL  20 . 00         4 . 00         Г     20 . 00 
 3 . 00         В     20 . 00         3 . 00         В     20 . 00 

( 5  row(s) affected)

alter table test1 add is_null smallint default  0 
go
update test1 set is_null =  1  where coalesce(f2,'NULL')='NULL'
go
create index idx_is_null on test1 (is_null)
go
create index idx_f2 on test1 (f2)
go
create index idx_f3 on test1 (f3)
go
create index idx_f12 on test2 (f12)
go
create index idx_f13 on test2 (f13)
go

select * from test1 t1, test2 t2
where (t1.f2 = t2.f12 and t1.f3 = t2.f13)
    or ( t1.f3 = t2.f13 and t1.is_null =  1 )