MYSQL在同一臺

Question

JOIN我現在有這個疑問設置：

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' 

這是因爲我想在一列從同一個表合併兩行。這是表

----------------------------------------------------------- 
| 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 | 
--------------------------------- 

基本上我想選擇所有主題的簡化設置，那麼也會返回如果有相應的圖像。我當前的查詢只返回所有與關聯圖像相關的主題。我嘗試了左和右外連接，但它似乎沒有幫助。

Answer 1

在OUTER JOIN上過濾時，必須在ON子句中或作爲派生表進行過濾。當image.display_order = '1'是在哪兒，它都永遠是一個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')