Oracle 11g Release 2

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CREATE TABLE tablename(ColumnA varchar2(21), ColumnB varchar2(18));
INSERT ALL 
    INTO tablename (ColumnA, ColumnB)
         VALUES ('a1', 'b1')
    INTO tablename (ColumnA, ColumnB)
         VALUES ('a1', 'b2')
    INTO tablename (ColumnA, ColumnB)
         VALUES ('a2', 'b1')
    INTO tablename (ColumnA, ColumnB)
         VALUES ('a2', 'b3')
    INTO tablename (ColumnA, ColumnB)
         VALUES ('a3', 'b2')
SELECT * FROM dual
;
5 rows affected

select t.*
from tablename t
where not exists (select 1 from tablename where ColumnA = t.ColumnA and ColumnB not in ('b1', 'b2'))


COLUMNA
COLUMNB


a3
b2

a1
b2

a1
b1


select distinct t.ColumnA
from tablename t
where not exists (select 1 from tablename where ColumnA = t.ColumnA and ColumnB not in ('b1', 'b2'))


COLUMNA


a1

a3