Question

长话短说，我正试图让我的网站上次更新时间。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

..这给了我类似的东西..


-------------------+-------------------+-------------------+-------------------
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 - 我现在如何获得最新的这些字段？要获得整个“网站上次更新”的时间。

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

Answer 2

在返回的字段上使用GREATEST()（如果您希望NULL值，则在通过COALESCE()之后）。

Answer 3

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

Answer 4

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

嵌套嵌套？ MySQL查询帮助

4 个答案: