mysql - 嵌套嵌套？ MySQL查询帮助

Question

长话短说，我正在尝试获取我的网站上次更新的时间。我站点的主要内容存储在四个表中。这些表中的每一个都有一个 updated_at 字段。

我可以在嵌套查询中组合它们:

SELECT (
    SELECT updated_at FROM proposition ORDER BY updated_at DESC LIMIT 1
) as latest_proposition,
(
    SELECT updated_at FROM product ORDER BY updated_at DESC LIMIT 1
) as latest_product,
(
    SELECT updated_at FROM review ORDER BY updated_at DESC LIMIT 1
) as latest_review,
(
    SELECT updated_at FROM segment ORDER BY updated_at DESC LIMIT 1 
) as latest_segment

..这给了我类似..

-------------------+-------------------+-------------------+-------------------
latest_proposition |latest_product     |latest_review      |latest_segment     
-------------------+-------------------+-------------------+-------------------
2011-05-25 14:45:30|2011-05-25 14:37:28|2011-05-12 09:20:19|2011-05-12 09:20:19
-------------------+-------------------+-------------------+-------------------
1 row in set (0.00 sec)

我的问题是 - 我现在如何获取这些字段中的最新信息？获取整体“网站上次更新”时间。

提前致谢!

Answer 1

select
    max(updated_at) as updated_at
from
(
    SELECT max(updated_at) as updated_at FROM proposition
    UNION
    SELECT max(updated_at) FROM product
    UNION
    SELECT max(updated_at) FROM review
    UNION
    SELECT max(updated_at) FROM segment
) x