Question

修改了

* ，但仍未解决...... *

我想要合并来自两个文件的数据，但我无法表达排序。我想排序第一个文件的CREATED日期和第二个文件的SHARED_DATE（如果它存在且更大）。

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

另一种可能性..

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

除了mySQL似乎不喜欢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

特别注意IS NOT而不是＆lt;＆gt;。 NULL在SQL中非常非常特殊。

Answer 2

SELECT n.created, s.shared_date,

(CASE WHEN s.shared_date IS NOT NULL
   THEN 
       (CASE WHEN s.shared_date > n.created 
       THEN s.shared_date ELSE n.created END)
   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

现在它应该像你想要的那样工作;）

mysql - JOIN或UNION - 不确定哪个，也许两个（第二部分）

2 个答案: