獲取給定時間段內分組項目的最新值，零和空值

Question

請你幫我找出我是否正確？我是SQL新手，所以有很多事情對我來說並不明顯。

我們假設我有兩張表。
首先是用戶的列表：

 subscribers 
+--------+------------+ 
|subscID | name | 
+--------+------------+ 
| 123 | SomeName00 | 
| 456 | SomeName01 | 
| 789 | SomeName02 | 
| 012 | SomeName03 | 
| 345 | SomeName04 | 
+--------+------------+ 

而第二個是呼叫日誌（或類似的東西），其中包括用戶的收入與事務ID的支出及其賬戶的當前狀態：

     transactions 
+--------+---------------------+--------+-----------+----------+ 
| trnID | date    |subscID | amount | balance | 
+--------+---------------------+-------+------------+----------+ 
| 321456 | 2012-03-13 11:10:00 | 456 | 70.0000 | 90.0000 | 
| 234567 | 2012-03-16 15:05:00 | 456 | -45.0000 | 45.0000 | 
| 345678 | 2012-03-19 17:27:00 | 456 | 15.0000 | 60.0000 | 
| 654321 | 2012-04-22 17:34:00 | 456 | -10.0000 | 50.0000 | 
|| 2012-04-15 15:45:00 | 789 | 20.0000 | 30.0000 | 
| 567890 | 2012-05-16 13:30:00 | 789 | -10.0000 | 20.0000 | 
| 876543 | 2012-02-29 11:00:00 | 012 | 20.0000 | 5.0000 | 
| 678901 | 2012-03-31 09:40:00 | 012 | 10.0000 | 15.0000 | 
| 456789 | 2012-03-31 21:09:00 | 012 | -13.0000 | 2.0000 | 
|9 | 2012-02-23 14:01:00 | 345 | -30.0000 | 27.0000 | 
|| 2012-03-24 19:57:00 | 345 | 40.0000 | 67.0000 | 
| 765432 | 2012-03-27 13:28:00 | 345 | -14.0000 | 53.0000 | 
+--------+---------------------+--------+-----------+----------+ 

---

起初我有兩個任務：

1.計算交易的數量每個用戶的 2012（包括那些從第一個表誰在 第二個表都沒有，像123 SomeName00月，和那些誰沒有在2012年3月任 交易，如789 SomeName02）。

2.要計數每個用戶的期末餘額爲2012年3月 （同樣，包括那些從第一個表誰在 第二個表不和那些誰沒有在2012年3月 任何交易）。

---

我已經處理的第一個是這樣的：

SELECT name, COUNT(transactions.subscID) AS num_of_trns 
FROM subscribers 
LEFT JOIN transactions 
ON subscribers.subscID = transactions.subscID 
    AND transactions.date LIKE "2012-03%" 
GROUP BY subscribers.subscID 

看來工作，給出以下結果：

+------------+------------+ 
| name |num_of_trns | 
+------------+------------+ 
| SomeName00 |  0  | 
| SomeName01 |  3  | 
| SomeName02 |  0  | 
| SomeName03 |  2  | 
| SomeName04 |  2  | 
+------------+------------+ 

然後我試圖重用代碼，以便通過將COUNT(transactions.subscID)更改爲transactions.balance並向LEFT JOIN添加一個條件（transactions.date = MAX(transactions.date)）來解決第二個任務，如下所示：

SELECT name, transactions.balance AS trns_blnc 
FROM subscribers 
LEFT JOIN transactions 
ON subscribers.subscID = transactions.subscID 
    AND transactions.date LIKE "2012-03%" 
    AND transactions.date = MAX(transactions.date) --this is incorrect 
GROUP BY subscribers.subscID 

但是這種方法證明是絕對錯誤的（我想，MySQL只是不明白我試圖比較哪些值）。

然後我決定利用第一個任務結果，再次將另一個transactions實例加入到它（如果我使用錯誤的術語），並且條件是實例的日期=舊錶的日期：

SELECT march_trns.name, balance FROM (
    SELECT name, date 
    FROM subscribers 
    LEFT JOIN transactions 
    ON subscribers.subscID = transactions.subscID 
     AND transactions.date LIKE "2012-03%" 
     GROUP BY subscribers.subscID 
) AS march_trns 
LEFT JOIN transactions AS transactions2 
ON march_trns.date = transactions2.date 

但很快就發現，march_trns.date和，因此，balance隨機選擇（或者我只是沒能找出其選擇的任何模式）的值中，滿足條件LIKE "2012-03%"。此外，我的結果表中有NULL（我猜，這是因爲我不再使用COUNT來計算所有行，包括。空值）：

  Have:       Want: 
+------------+----------+  +------------+----------+ 
| name  | balance |  | name  | balance | 
+------------+----------+  +------------+----------+ 
| SomeName00 |  NULL |  | SomeName00 | 0.0000 | 
| SomeName01 | 90.0000 |  | SomeName01 | 60.0000 | 
| SomeName02 |  NULL |  | SomeName02 | 0.0000 | 
| SomeName03 | 2.0000 |  | SomeName03 | 2.0000 | 
| SomeName04 | 67.0000 |  | SomeName04 | 53.0000 | 
+------------+----------+  +------------+----------+ 

所以，我有兩個問題：

• 我需要檢索月末餘額值
• ，並打印出NULL值0秒。

如果你告訴我正確的方向，我會非常感激你。

Answer 1

SELECT name, 
     COALESCE(COUNT(transactions.subscID), 0) AS num_of_trns, 
     COALESCE(d.balance,0) 
FROM subscribers 
     LEFT JOIN transactions 
      ON subscribers.subscID = transactions.subscID 
       AND transactions.date LIKE "2012-03%" 
     LEFT JOIN 
     (
      SELECT a.subscID, a.balance 
      FROM transactions a 
        INNER JOIN 
        (
         SELECT subscID, MAX(date) maxDate 
         FROM transactions 
         WHERE transactions.date LIKE "2012-03%" 
         GROUP BY subscID 
        ) b ON a.subscID = b.subscID AND 
          a.date = b.maxDate 

     ) d ON subscribers.subscID = d.subscID 
GROUP BY subscribers.subscID 
ORDER BY Name 

• SQLFiddle Demo

或通過使用YEAR和MONTH

SELECT name, 
     COALESCE(COUNT(transactions.subscID), 0) AS num_of_trns, 
     COALESCE(d.balance,0) 
FROM subscribers 
     LEFT JOIN transactions 
      ON subscribers.subscID = transactions.subscID 
       AND YEAR(transactions.date) = 2012 AND 
        MONTH(transactions.date) = 3 
     LEFT JOIN 
     (
      SELECT a.subscID, a.balance 
      FROM transactions a 
        INNER JOIN 
        (
         SELECT subscID, MAX(date) maxDate 
         FROM transactions 
         WHERE YEAR(transactions.date) = 2012 AND 
           MONTH(transactions.date) = 3 
         GROUP BY subscID 
        ) b ON a.subscID = b.subscID AND 
          a.date = b.maxDate 

     ) d ON subscribers.subscID = d.subscID 
GROUP BY subscribers.subscID 
ORDER BY Name 

• SQLFiddle Demo

SOURCE

• COALESCE
• MONTH
• YEAR