I agree with the other posters that it's not quite clear what you are trying to achieve, but I think a simple CROSS APPLY will give you the result you want (at least it gives me the same result set as your queries).

SELECT TOP 100

c.comp_id,

c.sup_code,

c.sup_name,

c.sup_ac_code,

s.site_id AS ac_id1

FROM

COMP_AC c

CROSS APPLY

SITE_AC s

WHERE

s.site_id = '0a1903120217062077241419'

ORDER BY

c.comp_id