一般来说，不要这样做，因为这样做会导致ticket_sold列很快与ticket表不同步。
如果您想要动态更新数据行，请执行下列动作：

1.使用视图。

您可以在需要时计算该值：

CREATE VIEW circus_view (circus_id, circus_date, circus_show_price, tickets_sold) AS
SELECT c.circus_id,
       c.circus_date,
       c.circus_show_price,
       (SELECT COUNT(*) FROM ticket t WHERE t.circus_id = c.circus_id)
FROM   circus c;

2.使用触发器。

如果必须在circus表中保留票证数，则：

ALTER TABLE Circus ADD tickets_sold NUMBER;

CREATE TRIGGER circus_tickets
  AFTER INSERT OR UPDATE OR DELETE ON Ticket
BEGIN
  UPDATE Circus c
  SET tickets_sold = (SELECT COUNT(*) FROM ticket t WHERE t.circus_id = c.circus_id);
END;
/

fiddle

Is is not group by clause you need because query then returns number of tickets per each circus, but - then you get as many rows as there are circus_id s in the ticket table. Instead, correlate subquery to the main table:

update circus c set 
  c.ticket_sold = (select count(t.ticket_id) 
                   from ticket t
                   where t.circus_id = c.circus_id
                  );