Oops, didn't notice it was a self join.

What I now think you want, is

select t.plantkey, t.mostrecent, t.tagnumber, t.datetested, t.equipmentkey
from reliefd t
where t.mostrecent='T' and
t.PlantKey='20030319103909097704' and
exists(select * from reliefd o where t.equipmentkey=o.equipmentkey and o.datetested<='2005')
order by t.tagnumber

I guess you don't like subselects (although I don't understand why), using a CTE would get you the same result:

WITH tmp(equipmentkey)
AS
(SELECT DISTINCT equipmentkey FROM reliefd
WHERE datetested <= '2005')
SELECT t.plantkey, t.mostrecent, t.tagnumber, t.datetested, t.equipmentkey
FROM reliefd t
JOIN tmp o ON t.equipmentkey = o.equipmentkey
WHERE t.mostrecent='T' AND
t.PlantKey='20030319103909097704'

HTH,
Set