MySQL Group by Week numw/複数の日付列

試してみてください：

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

sqlFiddle

約30（日付列）の左結合を書き込む必要があります。要件は、最初の日付列に空の週（カウント0）を含めないことです。そうしないと、結合が失われます。