Nested Nested? MySQL query help

Long story short, I'm trying to get the time my site was last updated. The main content of my site is stored in four tables. Each of 开发者_如何学运维these tables has an updated_at field.

I'm able to combine them in a nested query:

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

..which gives me something like..

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

My question is - How do I now get the latest of these fields? To get the overall "site last updated" time.

Thanks in advance!

---

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

---

Use GREATEST() on the returned fields (after passing them through COALESCE(), if you expect NULL values).

---

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