嵌套嵌套？ MySQL查詢幫助

Question

長話短說，我試圖讓我的網站上次更新的時間。 我的網站的主要內容存儲在四個表格中。 每個表都有一個updated_at字段。

我能夠向他們嵌套查詢相結合：

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 

..這讓我有點像..

-------------------+-------------------+-------------------+------------------- 
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) 

我的問題是 - 如何我現在得到的最新這些領域的？獲取整個「網站上次更新」時間。

在此先感謝！

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

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 3

使用GREATEST()在返回的字段（將它們通過COALESCE()後，如果您希望NULL值）。

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