SQL 1/4 3. Как сочетать объединения и подзапросы

SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS MONTH,
       COUNT(invoice_id) AS year_2011
FROM invoice
WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) = 2011
--OR EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2012
--OR EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2013
GROUP BY MONTH

UNION ALL

SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS MONTH,
       COUNT(invoice_id) AS year_2012
FROM invoice
--WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2011
WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) = 2012
--OR EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2013
GROUP BY MONTH

UNION ALL

SELECT EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) AS MONTH,
       COUNT(invoice_id) AS year_2013
FROM invoice
--WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2011
--WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2012
WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) = 2013
GROUP BY MONTH