如何計算一段時間內的平均值？

Question

我想在一段時間內（例如：過去6個月，上個月，上週...）有平均水平。

爲此，我創建了4個查詢。我能清楚地做點什麼嗎？

/** ON SELECTED DATE*/ 
SELECT AVG(RUN_TIME) 
FROM (SELECT (DATEDIFF(minute,MIN(CAST(START_DATE AS DATETIME)), MAX(CAST(END_DATE AS DATETIME)))) as RUN_TIME FROM TableTest where DATE_EXPECTED = DATE_SELECTED) f 

/**ON 6 last month*/ 
SELECT AVG(RUN_TIME) 
FROM (SELECT (DATEDIFF(minute,MIN(CAST(START_DATE AS DATETIME)), MAX(CAST(END_DATE AS DATETIME)))) as RUN_TIME FROM TableTest 
where Month(CAST(DATE_SELECTED AS DATETIME)) BETWEEN Month(CAST(DATE_SELECTED AS DATETIME)) AND Month(CAST(DATE_SELECTED AS DATETIME))+6) f 

/** on 30 last days */ 
SELECT AVG(RUN_TIME) 
FROM (SELECT (DATEDIFF(minute,MIN(CAST(START_DATE AS DATETIME)), MAX(CAST(END_DATE AS DATETIME)))) as RUN_TIME FROM TableTest where DATE_EXPECTED between DATE_SELECTED and DATE_SELECTED+30) f 

/* on 5 last days */ 
SELECT AVG(RUN_TIME) 
FROM (SELECT (DATEDIFF(minute,MIN(CAST(START_DATE AS DATETIME)), MAX(CAST(END_DATE AS DATETIME)))) as RUN_TIME FROM TableTest where DATE_EXPECTED between DATE_SELECTED and DATE_SELECTED+5) f 

它的工作，但我想知道如果我能以最好的方式做這件事。

感謝，

Answer 1

可以使用條件彙總：

SELECT AVG(CASE WHEN DATE_EXPECTED = DATE_SELECTED THEN RUN_TIME END) as avg_1, 
     AVG(CASE WHEN Month(CAST(DATE_SELECTED AS DATETIME)) BETWEEN Month(CAST(DATE_SELECTED AS DATETIME)) AND Month(CAST(DATE_SELECTED AS DATETIME))+6 THEN RUN_TIME END) as avg_2, 
     AVG(CASE WHEN DATE_EXPECTED between DATE_SELECTED and DATE_SELECTED+30 THEN RUN_TIME END) as avg_3, 
     AVG(CASE WHEN DATE_EXPECTED between DATE_SELECTED and DATE_SELECTED+5 THEN RUN_TIME END) as avg_4 
FROM (SELECT (DATEDIFF(minute,MIN(CAST(START_DATE AS DATETIME)), MAX(CAST(END_DATE AS DATETIME)))) as RUN_TIME 
     FROM TableTest 
    ) f;