задача 1

SELECT aa.invoice_month,
       aa.year_2011,
       bb.year_2012,
       cc.year_2013
FROM
(SELECT EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) AS invoice_month,
       COUNT(invoice_id) AS year_2011
FROM invoice AS i
WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) = '2011'
GROUP BY EXTRACT(MONTH FROM CAST(invoice_date AS timestamp))
ORDER BY invoice_month) AS aa
LEFT OUTER JOIN
(SELECT EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) AS invoice_month,
       COUNT(invoice_id) AS year_2012
FROM invoice AS i
WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) = '2012'
GROUP BY EXTRACT(MONTH FROM CAST(invoice_date AS timestamp))
ORDER BY invoice_month) AS bb
ON aa.invoice_month=bb.invoice_month
LEFT OUTER JOIN
(SELECT EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) AS invoice_month,
       COUNT(invoice_id) AS year_2013
FROM invoice AS i
WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) = '2013'
GROUP BY EXTRACT(MONTH FROM CAST(invoice_date AS timestamp))
ORDER BY invoice_month)) AS cc
ON aa.invoice_month=cc.invoice_month;