probeer:
SELECT weekofyear(forcast_date) AS week_forcast,
COUNT(forcast_date) AS count_forcast, t2.count_actual
FROM
t t1 LEFT JOIN (
SELECT weekofyear(actual_date) AS week_actual,
COUNT(forcast_date) AS count_actual
FROM t
GROUP BY weekOfYear(actual_date)
) AS t2 ON weekofyear(forcast_date)=week_actual
GROUP BY
weekofyear(forcast_date), t2.count_actual
Je moet ongeveer 30 (je datumkolommen) overblijven om mee te doen, en de vereiste is dat je eerste datumkolom een lege week zou moeten hebben (met een telling van 0) of de deelnames zullen missen.