MySQL的組條件

Question

想我已經得到了如下表：

time_recorded  | room | num_occupied 
---------------------------------------------- 
2012-11-05 00:00:00  a   10 
2012-11-05 00:00:00  b   20 
2012-11-05 00:00:00  c   30 
2012-11-05 00:00:00  d   40 

2012-11-05 00:30:00  a   100 
2012-11-05 00:30:00  b   200 
2012-11-05 00:30:00  c   300 
2012-11-05 00:30:00  d   400 
        ... 

我需要組行時房間=「b」和「C」，從而使輸出結果將是

time_recorded  | room | num_occupied 
---------------------------------------------- 
2012-11-05 00:00:00  a   10 
2012-11-05 00:00:00  bc  (20+30) 
2012-11-05 00:00:00  d   40 

2012-11-05 00:30:00  a   100 
2012-11-05 00:30:00  bc  (200+300) 
2012-11-05 00:30:00  d   400 
        ... 

我嘗試下面的查詢，通過克林斯曼d建議：

SELECT 
    * 
FROM 
    usage 
WHERE 
    DATE(time_recorded) = '2012-11-05' 
    GROUP BY CASE WHEN room IN ('b', 'c') THEN 'bc' ELSE room END; 

出於某種原因，我得到以下返回：

time_recorded  | room | num_occupied 
---------------------------------------------- 
2012-11-05 00:00:00  a   10 
2012-11-05 00:00:00  b   20 

我在做什麼錯了？

預先感謝您！

Answer 1

您正在按計算值分組，但未顯示該值。

您需要將CASE表達式放在SELECT子句中，同時還需要計算字段以顯示num_occupied的值。事情是這樣的：

SELECT 
    time_recorded, CASE WHEN room IN ('b', 'c') THEN 'bc' ELSE room END, 
    SUM(num_occupied) 
FROM 
    usage 
WHERE 
    DATE(time_recorded) = '2012-11-05' 
    GROUP BY 1, 2; 

（根據註釋編輯）