SQL JOIN從第二臺

Question

我有一個表Mark與id和name場和第二表mark_id和兩個額外的列我感興趣的是獲得不同列中。我想從Mark表+優先適用於所有記錄標記和created_at。可能有幾個值，但我不太在乎它們，我只需要一個（首先，不管）。

我試過這個查詢，它給了我太多的記錄，5而不是3我期望。

SELECT 
    marks.*, 
    markings.created_at, 
    markings.created_at AS updated_at, 
    markings.priority 
FROM `marks` 
JOIN markings 
    ON marks.id = markings.tag_id 
WHERE (created_at > '2011-11-06 12:05:01') 
    AND (priority = 1) 
ORDER BY created_at; 

Answer 1

SELECT 
    marks.*, 
    MIN(markings.created_at) as created_at, 
    MIN(markings.created_at) AS updated_at, 
    markings.priority 
FROM `marks` 
JOIN markings 
    ON marks.id = markings.tag_id 
WHERE (created_at > '2011-11-06 12:05:01') 
    AND (priority = 1) 
GROUP BY marks.col1, marks.col2, ... marks.coln, markings.priority 
ORDER BY created_at; 

其中marks.col1, marks.col2, ... marks.coln將你的情況marks.id, marks.name如果你沒有更多的列你不告訴我們。

Answer 2

此查詢組通過標誌和編號的結果，應該是你想要的東西：

SELECT 
    marks.id, 
    MAX(markings.created_at), 
    MAX(markings.created_at) AS updated_at, 
    markings.priority 
FROM `marks` 
JOIN markings 
    ON marks.id = markings.tag_id 
WHERE (created_at > '2011-11-06 12:05:01') 
    AND (priority = 1) 
GROUP BY marks.id,markings.priority 
ORDER BY updated_at;