Nested Nested? Aide sur les requêtes MySQL

Question

Bref, j'essaie de connaître l'heure de la dernière mise à jour de mon site. Le contenu principal de mon site est stocké dans quatre tableaux. Chacune de ces tables possède un champ updated_at.

Je suis en mesure de les combiner dans une requête imbriquée:

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 me donne quelque chose comme ..

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

Ma question est - Comment puis-je obtenir maintenant la dernière de ces champs? Pour obtenir l'heure globale "mise à jour du site".

Merci d'avance!

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

Utilisez GREATEST() sur les champs renvoyés (après passage à travers COALESCE(), si vous vous attendez NULL valeurs).

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