Почему не работает код: SELECT last_name FROM (SELECT DISTINCT t_3.month FROM (SELECT customer_id AS month FROM invoice WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2013 AND EXTRACT(MONTH FROM CAST(invoice_date AS date)) = 1) AS t_3) AS t_1 JOIN (SELECT c.customer_id AS month FROM client AS c LEFT JOIN invoice AS i ON i.customer_id = c.customer_id WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2013 AND EXTRACT(MONTH FROM CAST(invoice_date AS date)) > 1) AS t_2 ON t_1.month = t_2.month

Код не работает из-за ошибки в выражении EXTRACT(MONTH FROM CAST(invoice_date AS date)) > 1. Здесь символ > является HTML-кодом для знака "больше чем" (>). Вместо этого выражения следует использовать обычное сравнение EXTRACT(MONTH FROM CAST(invoice_date AS date)) > 1.

Исправленный код будет выглядеть следующим образом:

SELECT last_name FROM (SELECT DISTINCT t_3.month FROM (SELECT customer_id AS month FROM invoice WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2013 AND EXTRACT(MONTH FROM CAST(invoice_date AS date)) = 1) AS t_3) AS t_1 JOIN (SELECT c.customer_id AS month FROM client AS c LEFT JOIN invoice AS i ON i.customer_id = c.customer_id WHERE EXTRACT(YEAR FROM CAST(invoice_date AS date)) = 2013 AND EXTRACT(MONTH FROM CAST(invoice_date AS date)) > 1) AS t_2 ON t_1.month = t_2.month

После внесения этой коррекции код должен работать правильно.