create table B (
BID int not null primary key
);

create table RType (
RTipeID int not null primary key
);


insert into RType (RTypeID) values ( 1 ); -- 1-1
insert into RType (RTypeID) values ( 2 ); -- M-N

create table A (
AID int not null primary key,
BID int not null,
foreign key (BID) references B (BID)
);

create table AB (
ABID  int not null primary key,
AID int not null,
BID int not null,
RTypeID int not null, 
foreign key (AID) references A (AID),
foreign key (BID) references B (BID),
foreign key (RTypeID) references RTipe (RTypeID)
);
go

create unique index IX_AB_11 on AB (AID) where RTypeID= 1 ;
go

create unique index IX_AB_MN on AB (AID, BID);
go

create trigger TG_AB_IU on AB for insert, update as
begin
 if exists (select AID from A inner join inserted i on A.AID=i.AID and A.BID=i.BID)
 begin
  rollback transaction
 end
end
go