嵌套嵌套？ MySQL查询帮助

问题描述

| 长话短说，我正在尝试获取网站的最新更新时间。 我网站的主要内容存储在四个表中。 每个表都有一个updated_at字段。 我可以将它们合并到一个嵌套查询中：

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

..这给了我一些像..

-------------------+-------------------+-------------------+-------------------
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-现在如何获得这些领域的最新信息？获取整个“网站上次更新”时间。 提前致谢！     

解决方法

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

    ,在返回的字段上使用GREATEST（）（如果希望通过

NULL

值，则将它们通过COALESCE（）传递之后）。     ,

SELECT updated_at 
from 
(
    SELECT updated_at FROM proposition ORDER BY updated_at DESC LIMIT 1
          union all
    SELECT updated_at FROM product ORDER BY updated_at DESC LIMIT 1
          union all
    SELECT updated_at FROM review ORDER BY updated_at DESC LIMIT 1
          union all
    SELECT updated_at FROM segment ORDER BY updated_at DESC LIMIT 1 
) x
order by updated_at limit 1

    ,

SELECT MAX(updated_at)
FROM (SELECT MAX(updated_at) FROM proposition UNION ALL
  SELECT MAX(updated_at) FROM product UNION ALL
  SELECT MAX(updated_at) FROM review UNION ALL
  SELECT MAX(updated_at) FROM segment) x