SQL Server 2019

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create table AtoB (Aid int, Bid int);
insert into AtoB values (1,1),
(1,1),
(1,2),
(1,3),
(2,2),
(3,1),
(4,1),
(1,2),
(1,1),
(5,5)
10 rows affected

select * from AtoB ab
where Aid=1 and not exists (select * from AtoB ab2
where ab2.Aid<>1 and ab2.bid=ab.Bid)


Aid
Bid


1
3


select * from AtoB ab
where Aid=1 


Aid
Bid


1
1

1
1

1
2

1
3

1
2

1
1