$q = $this->db->select('
books.title,
reserved_books.id,
reserved_books.isbn,
reserved_books.price,
reserved_books.item_sold,
reserved_books.date_sold,
reserved_books.total_amount
')
->from('reserved_books')
->join('books','reserved_books.isbn= books.isbn')
->limit($limit,$offset);
我如何在查询中使用 distinct ?reserved_books.isbn 是唯一的。