Encuentre un grupo de registros que coincidan con múltiples valores

Puede hacer esto con la agregación condicional:

select parentid 
from tablename
group by parentid
having sum(case when datavalue = 1 then 1 else 0 end) > 0 and
       sum(case when datavalue = 6 then 1 else 0 end) > 0

Otra forma es usar exists :

select distinct parentid
from tablename t1
where exists(select * from tablename where parentid = t1.parentid and datavalue = 1) and
      exists(select * from tablename where parentid = t1.parentid and datavalue = 6)

Otra forma es contar ocurrencias distintas:

select parentid 
from tablename
where datavalue in(1, 6)
group by parentid
having count(distinct datavalue) = 2