Sql e FK至邮政；共享的顶层也会有相同的FK发布。请帮我理解为什么不是这样。 |----------------| |-------------------| |-------------------| | Post | |

e FK至邮政；共享的顶层也会有相同的FK发布。请帮我理解为什么不是这样。

|----------------|    |-------------------| |-------------------|
|      Post      |    |     Top_Level     | |      Comment      |   
|----------------|    |-------------------| |-------------------|   
| PK | ID        |    | PK, FK | Post_ID  | | PK, FK | Post_ID  |   
|    | DATE      |    |        | Title    | |     FK | TopLv_ID |   
|    | Text      |    |-------------------| |-------------------|   
|----------------|                                                  

|------------------------|    |------------------|    |--------------------|
|      Post              |    |     Top_Level    |    |       Comment      |
|------------------------|    |------------------|    |--------------------|
| ID |    DATE    | Text |    | Post_ID  | Title |    | Post_ID  |TopLv_ID |
|----|------------|------|    |----------|-------|    |----------|---------|
|  1 | 13/03/2008 | shy  |    |     1    |  XYZ  |    |     2    |    1    |
|  2 | 14/03/2008 | mrj  |    |     3    |  ABC  |    |     4    |    1    |
|  3 | 15/03/2008 | quw  |    |     7    |  NMO  |    |     5    |    3    |
|  4 | 16/03/2008 | ksi  |    |------------------|    |     6    |    1    |
|  5 | 17/03/2008 | kso  |                            |--------------------|
|  6 | 18/03/2008 | aoo  |                            
|  7 | 19/03/2008 | all  |                            
|------------------------|     

|--------------------------------|
|            RESULT              |
|--------------------------------|
| ID |    DATE    | Title | Text |
|----|------------|-------|------|
|  7 | 19/03/2008 |  123  | all  |
|  1 | 13/03/2008 |  ABC  | shy  |
|  3 | 15/03/2008 |  XYZ  | quw  |
|--------------------------------|

SELECT DISTINCT sub1.*, super.*

FROM super INNER JOIN sub1 ON super_id
    LEFT JOIN sub2 ON super_id AND sub1_id

ORDER BY COALESCE(sub2.dateColumn, sub1.dateColumn)

ORDER BY ISNULL(firstDateColumn, ISNULL(secondDateColumn, thirdDateColumn))

SELECT pt.id, pt.`date`, t.title, pt.`text`
FROM Top_Level t INNER JOIN Post pt ON (t.post_id = pt.id)
 LEFT OUTER JOIN (Comment c INNER JOIN Post pc ON (c.post_id = pc.id))
   ON (c.toplv_id = t.post_id)
GROUP BY pt.id
ORDER BY MAX(GREATEST(pt.`date`, pc.`date`)) ASC;

+----+------------+-------+------+
| id | date       | title | text |
+----+------------+-------+------+
|  7 | 2008-03-19 | NMO   | all  | 
|  1 | 2008-03-13 | XYZ   | shy  | 
|  3 | 2008-03-15 | ABC   | quw  | 
+----+------------+-------+------+