Jeg tror, at dette vil hjælpe dig:
SELECT A.id, A.col_1, A.col_2, A.datetime_col, A.col_3
FROM
(SELECT B.id, B.col_1, B.col_2, C.datetime_col, C.col_3
FROM tableA B LEFT OUTER JOIN tableB C ON B.id = C.id
ORDER BY C.datetime_col desc) as A
GROUP BY A.id