Question

* revised, but still not solved…*

I want to combine data from two files but I am having trouble expressing the sort. I want to sort on either the CREATED date from the first file and the SHARED_DATE from the second file if it exists and it is larger.

file 1 : NODE 
fields: CREATED, CHANGED

file 2 : SHARE_CONTENT   
fields:  SHARED_DATE ,  NODE (foreign key)

…

SELECT n.created, s.shared_date,

(CASE WHEN s.shared_date <> NULL
   then  s.shared_date
   else n.created
END) 
as lastshare

FROM node as n LEFT JOIN share_content as s on (s.nid=n.nid)

ORDER BY lastshare DESC

---

another possibility..

SELECT n.created, s.shared_date,
FROM node as n LEFT JOIN share_content as s on (s.nid=n.nid)
ORDER BY MAX(n.created, s.shared_date) DESC

except mySQL does not seem to like MAX(a,b)

Answer 1

SELECT n.created, s.shared_date,
(CASE WHEN (s.shared_date IS NOT NULL) And (s.Shared_Date > n.Created)
   then  s.shared_date
   else n.created
END) 
as lastshare
FROM node as n LEFT JOIN share_content as s on (s.nid=n.nid)
ORDER BY lastshare DESC

Specifically note the IS NOT instead of <>. NULL is very, very special in SQL.