Hi, at a guess I believe the code below should do what you require.
SELECT
t1.A
,t1.B
,t1.C
,t2.D
FROM
Table1 t1
LEFT OUTER JOIN Table2 t2 ON
t1.A = t2.A AND
t1.B = t2.B
I usually use a Left Outer Join onto the table I want a null to come from
So at a guess you will have NULL in D for all but 5 results.
Hope that helps