我可以使用從SELECT子句中的子查詢返回的列來對結果進行排序嗎？

Question

使用下面的示例，這不是確切的代碼，只是我試圖實現的一個示例：

該查詢將用於返回五個最常訪問的頁面，該頁面有多少次訪問註冊。

SELECT 
    page_instance.name AS 'page name', 

    /* Sub Query I wish to sort by */ 
    (
     SELECT COUNT(*) 
     FROM visitor_event 
     WHERE page_instance.id = visitor_event.item_id 
    ) AS 'visit count' 

    FROM item_event 

    LEFT JOIN visitor_event_type ON visitor_event_type.id = visitor_event_type_id 
    LEFT JOIN page_instance ON page_instance.id = visitor_event.item_id 
    LEFT JOIN page ON page.id = page_instance.page_id 

    WHERE visitor_event_type.handle = 'viewed' 
    AND page_instance.name != 'NULL' 
    GROUP BY page.id 
    ORDER BY 'visit count' DESC /* Where its going wrong */ 
    LIMIT 5 

當前，查詢運行時沒有錯誤，但不會按「訪問計數」列進行排序。

我見過類似的查詢，把子查詢放在FROM子句中。我不明白這個原因，因爲我可能想要其他子查詢（對於最大/最小結果等）的其他列。我不反對這個想法，我只是不明白其所以然它

Answer 1

這樣做：

SELECT temp.* FROM (SELECT 
    page_instance.name AS 'page name', 
    (
     SELECT COUNT(*) 
     FROM visitor_event 
     WHERE page_instance.id = visitor_event.item_id 
    ) AS 'visit_count' 

    FROM item_event 

    LEFT JOIN visitor_event_type ON visitor_event_type.id = visitor_event_type_id 
    LEFT JOIN page_instance ON page_instance.id = visitor_event.item_id 
    LEFT JOIN page ON page.id = page_instance.page_id 

    WHERE visitor_event_type.handle = 'viewed' 
    AND page_instance.name != 'NULL' 
    GROUP BY page.id 
    LIMIT 5) as temp ORDER BY temp.visit_count DESC