This is what I could do :
SELECT t2.at, t2.appointments, (@n := IFNULL(t1.engineers, @n)) FROM t2
LEFT JOIN (
SELECT t.at, t1.engineers
FROM t1
JOIN t2 t ON t1.at = t.at
) t1 ON t1.at = t2.at;
I am sure there is something better out there, as the redondant JOIN
could not be needed, but I could not find it.
It actually use a sql variable to get the last value if there is no corresponding entry in the engineers table.
Don't forget to run SET @n = 0;
.