SELECT SQL查詢（MySQL）中的計算列

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：

我要在此處添加另一列，即“ ExpiredCount = TotalCount-ActiveCount ”

我該如何實現？

Answer 1

您只需要一個表達式。 但是，您可以簡化查詢：

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;

筆記：

• 表別名應為表名稱的縮寫，因此易於閱讀。
• 你並不需要join此查詢，只是有條件的聚集。
• 不必要使用反引號，這會使查詢更難編寫。
• 您問題的答案很簡單。

編輯：

如果要將其用作視圖，則FROM子句中不能有子查詢（MySQL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

Answer 2

Gordon的答案可能是我希望看到的DBA繼承別人的代碼。 但是我們實際上可以完全避免使用子查詢，而只需編寫以下內容：

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