Как сочетать объединения и подзапросы, задача 1

/*Составьте сводную таблицу. Посчитайте заказы, оформленные за каждый месяц в течение нескольких лет: с 2011 по 2013 год. Итоговая таблица должна включать четыре поля: invoice_month, year_2011, year_2012, year_2013. Поле invoice_month должно хранить месяц в виде числа от 1 до 12.
Если в какой-либо месяц заказы не оформляли, номер такого месяца всё равно должен попасть в таблицу.
В этом задании не будет подсказок. Используйте любые методы, которые посчитаете нужными.
*/

SELECT i_month.invoice_month,
       inv_2011.year_2011,
       inv_2012.year_2012,
       inv_2013.year_2013
FROM
  (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS invoice_month
   FROM invoice
   GROUP BY invoice_month
   ORDER BY invoice_month) AS i_month


LEFT JOIN
  (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS invoice_month,
          COUNT(invoice_id) AS year_2011
   FROM invoice
   WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) = 2011
   GROUP BY invoice_month) AS inv_2011 ON i_month.invoice_month = inv_2011.invoice_month


LEFT JOIN
  (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS invoice_month,
          COUNT(invoice_id) AS year_2012
   FROM invoice
   WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) = 2012
   GROUP BY invoice_month) AS inv_2012 ON i_month.invoice_month = inv_2012.invoice_month


LEFT JOIN
  (SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS invoice_month,
          COUNT(invoice_id) AS year_2013
   FROM invoice
   WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) = 2013
   GROUP BY invoice_month) AS inv_2013 ON i_month.invoice_month = inv_2013.invoice_month