create table A1(
id int,
classId char(2),
name char(2)
)

insert into A1 values (1,1,‘A1‘)
insert into A1 values (1,2,‘A2‘)
insert into A1 values (2,2,‘A3‘)

create table B1(
id int,
classId char(2),
name char(2)
)

insert into B1 values (1,1,‘B1‘)
insert into B1 values (1,2,‘B2‘)
insert into B1 values (2,null,‘B3‘)



select A1.id Aid,A1.classId AClassId,A1.name A1name,B1.* into #aa from A1 inner join B1 on A1.id = B1.id and A1.classId = B1.classId

select * from #aa
union
select * from A1 inner join B1 on A1.id = B1.id and not exists(select * from #aa a where A1.id =a.Aid and A1.classId = a.AClassId )
    
drop table #aa
drop table A1
drop table B1