with t(a,b) as(
select 'FLAG','tang' from dual
union select 'FLAG','song' from dual
union select 'OTHER','song' from dual
)
select b from t where a='FLAG'
    and not exists (select * from t t1 where t1.b=t.b and t1.a<>'FLAG');
/*
B
------
tang
*/

------解决方案--------------------

不好意思发错了
SQL code

select a.字段A, a.字段B from tmptb a
where a.字段B not in (select 字段B from tmptb where 字段A <> 'FLAG') and a.字段A='FLAG'