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Для каждого года, выраженного числом, посчитайте уникальные названия купленных треков.

SELECT EXTRACT(YEAR FROM CAST(i.invoice_date AS date)) AS year_of_invoice,
       COUNT(DISTINCT t.name)
FROM invoice AS i-- определите левую таблицу
LEFT OUTER JOIN invoice_line AS il ON i.invoice_id = il.invoice_id
LEFT OUTER JOIN track AS t ON il.track_id = t.track_id
GROUP BY year_of_invoice;