SQL查詢幫助 - 子查詢/嵌入查詢

Question

我剛剛開始在工作中學習SQL，但我遇到了與subquerys有關的問題。 我希望不僅僅是一個查詢，而是解釋如何創建子查詢。

當前的SQL查詢

SELECT to_char(P_DT, 'yyyy-mm-dd HH:mi:ss'), COUNT, LATENCY 
FROM latency ORDER BY P_DT 

返回

to_char(P_DT, 'yyyy-mm-dd HH:mi:ss'), COUNT, LATENCY 
2012-04-20 03:42:00  83659 1m 
2012-04-20 03:42:00 70305 2m 
2012-04-20 03:42:00 105194 4m 
2012-04-20 03:43:00 70277 1m 
2012-04-20 03:43:00 0 2m 
2012-04-20 03:43:00 0 4m 

我需要什麼

to_char(P_DT, 'yyyy-mm-dd HH:mi:ss'), COUNT.LATENCY='1m', COUNT.LATENCY='2m', COUNT.LATENCY='4m' 
2012-04-20 03:42:00 83659  70305  105194 
2012-04-20 03:43:00 70227  0   0 

感謝

數據庫有3列：時間戳，計數，延遲型

Tmestamp = progressive timestamp for each min 
Count = count, just a number 
Latency Type = 1m, 2m or 4m 

每個時間戳都有1m，2m和4m延遲，但這些都是不同的行。

我需要返回一行是 時間戳，計算在時間戳1M，計算在時間戳2M，在時間戳

計數4米我無法改變的，而不是count1m列count2m列的數據庫等

看它是這三個查詢合併爲一個

SELECT to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\'), COUNT 
FROM LATENCY 
WHERE SOURCE_ID=\'2\' AND LATENCY = \'1m\' 
GROUP BY to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\') 
ORDER BY to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\') 

SELECT to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\'), COUNT 
FROM LATENCY 
WHERE SOURCE_ID=\'2\' AND LATENCY = \'2m\' 
GROUP BY to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\') 
ORDER BY to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\') 

SELECT to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\'), COUNT 
FROM LATENCY 
WHERE SOURCE_ID=\'2\' AND LATENCY = \'4m\' 
GROUP BY to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\') 
ORDER BY to_char(P_DT, \'yyyy-mm-dd HH:mi:ss\') 

Answer 1

如果我理解正確的，你一個不同的方式，這應該這樣做：

SELECT to_char(P_DT, 'yyyy-mm-dd HH:mi:ss'), 
     SUM(CASE WHEN LATENCY = '1m' THEN COUNT ELSE 0 END) AS LATENCY1m, 
     SUM(CASE WHEN LATENCY = '2m' THEN COUNT ELSE 0 END) AS LATENCY2m, 
     SUM(CASE WHEN LATENCY = '4m' THEN COUNT ELSE 0 END) AS LATENCY4m 
FROM latency 
GROUP BY to_char(P_DT, 'yyyy-mm-dd HH:mi:ss') 
ORDER BY to_char(P_DT, 'yyyy-mm-dd HH:mi:ss')