您可以建立检视：

CREATE VIEW vwlocations 
(
    location_id,
    street_address,
    postal_code,
    city ,
    state_province,
    country_id,
    nb_departments
) AS
SELECT  l.location_id,
        l.street_address,
        l.postal_code,
        l.city ,
        l.state_province,
        l.country_id,
        COUNT(d.location_id) AS nb_departments
FROM    locations l
        JOIN departments d
            ON l.location_id = d.location_id
GROUP BY l.location_id,
        l.street_address,
        l.postal_code,
        l.city ,
        l.state_province,
        l.country_id

如果确实要向现有表中添加列，则可以通过以下方式初始化列值，然后添加一个触发器来管理更新/插入/删除：

UPDATE locations SET locations.nb_departments = (SELECT COUNT(*) FROM departments WHERE locations.location_id = departments.location_id)