SELECT t1.A, t1.B, t1.C, t2.D
FROM t1
LEFT JOIN t2
ON t1.A = t2.A AND t1.B = t2.B AND t1.C = t2.C
Try this and check if you like the results, if not, use this (psedo.code):
insert into #temptable
SELECT t1.A, t1.B, t1.C, t2.D
FROM t1, t2
WHERE t1.A = t2.A AND t1.B = t2.B AND t1.C = t2.C
IF select count(*) from #temptable = 0 then
insert into #temptable
SELECT t1.A, t1.B, t1.C, t2.D
FROM t1, t2
WHERE t1.A = t2.A AND t1.B = t2.B
end if;
IF select count(*) from #temptable = 0 then
insert into #temptable
SELECT t1.A, t1.B, t1.C, t2.D
FROM t1, t2
WHERE t1.A = t2.A
end if;
select * from #temptable
drop #temptable