計算列

Question

我有以下查詢：

SELECT 
a.source, 
TotalCount, 
ActiveCount 
FROM 
(
    SELECT COUNT(*) AS TotalCount, a.source 
    FROM internship a 
    GROUP BY a.source 
) AS A 
join 
(
    SELECT COUNT(*) AS ActiveCount, b.source 
    FROM internship b 
    WHERE b.int_lastdate > CURDATE() AND b.status LIKE 'Published' 
    GROUP BY b.source 
) AS B 
    ON A.source = B.source 

上面的查詢給了我這樣的結果：

我想補充另一列這將是「ExpiredCount = TotalCount - ActiveCount」

我該如何達到這個目標？

Answer 1

戈登的答案可能是什麼，我會希望看作是繼承別人代碼的DBA。但是我們實際上可以避免使用子查詢，只需寫下以下內容：

SELECT 
    source, 
    COUNT(*) AS TotalCount, 
    SUM(CASE WHEN int_lastdate > CURDATE() AND status LIKE 'Published' 
      THEN 1 ELSE 0 END) AS ActiveCount, 
    COUNT(*) - SUM(CASE WHEN int_lastdate > CURDATE() AND status LIKE 'Published' 
         THEN 1 ELSE 0 END) AS ExpiredCount 
FROM internship 
GROUP BY source 

Answer 2

你只需要一個表達式。但是，您可以簡化您的查詢：

select i.source, i.TotalCount, i.ActiveCount, 
     (i.TotalCount - i.ActiveCount) as ExpiredCount 
from (select i.source, count(*) as TotalCount, i.source, 
      sum(i.int_lastdate > curdate() and i.status like 'Published') as ActiveCount 
     from internship i 
     group by i.source 
    ) i; 

注：

• 表的別名應該是表名的縮寫，所以他們很容易閱讀。
• 對於此查詢，您不需要join，只是有條件的聚合。
• 過度使用反引號是不必要的，使查詢更難寫。
• 您的問題的答案是一個簡單的表達式。

編輯：

如果你想用這個作爲一個視圖，你不能有FROM子句中的子查詢（MySQL的限制）：

 select i.source, count(*) as TotalCount, i.source, 
      sum(i.int_lastdate > curdate() and i.status like 'Published') as ActiveCount, 
      (count(*) - sum(i.int_lastdate > curdate() and i.status like 'Published')) as ExpiredCount 
     from internship i 
     group by i.source