MYSQL JOIN 同一张表

Question

我目前有这样的查询设置：

SELECT 
  topic.content_id, 
  topic.title, 
  image.location 
FROM 
  mps_contents AS topic 
  INNER JOIN mps_contents AS image 
    ON topic.content_id = image.page_id 
WHERE 
  topic.page_id = (SELECT page_id FROM mps_pages WHERE page_short_name = 'foo' ) 
  AND image.display_order = '1'

这是因为我想将同一个表中的两行合并为一行。这是表格的简化设置

-----------------------------------------------------------
| page_id | content_id | title | location | display_order |
-----------------------------------------------------------
|    1    |     200    |  Foo  |   NULL   |     200       |
|    1    |     201    |  Baz  |   NULL   |     201       |
|   200   |     201    |  Bar  | jpg.jpg  |      1        |
-----------------------------------------------------------

And basically I want this result

---------------------------------
| content_id | title | location |
---------------------------------
|     200    |  Foo  | jpg.jpg  |
|     201    |  Baz  |   NULL   |
---------------------------------

基本上我想选择所有主题，然后还返回相应的图像（如果有）。我当前的查询仅返回具有关联图像的所有主题。我尝试了左外连接和右外连接，但似乎没有帮助。

Answer 1

当您对 OUTER JOIN 进行过滤时，您必须在 ON 子句中或作为派生表进行过滤。当 image.display_order = '1' 位于 WHERE 中时，它将始终是 INNER JOIN

SELECT 
  topic.content_id, 
  topic.title, 
  image.location 
FROM 
  mps_contents AS topic 
  LEFT JOIN
  mps_contents AS image ON topic.content_id = image.page_id
             AND image.display_order = '1'  
WHERE 
  topic.page_id = (SELECT page_id FROM mps_pages WHERE page_short_name = 'foo' ) 

或

SELECT 
  topic.content_id, 
  topic.title, 
  image.location 
FROM 
  mps_contents AS topic 
  LEFT JOIN
  (
   SELECT *
   FROM mps_contents
   WHERE display_order = '1'
  ) AS image ON topic.content_id = image.page_id
WHERE 
  topic.page_id = (SELECT page_id FROM mps_pages WHERE page_short_name = 'foo' )