如何使此MySQL查詢更有效？ 子查詢/左聯接

Question

我正在尋找有關快速查詢的幫助-

SELECT DISTINCT waits.ride, 
                T2.daywait, 
                T3.timewait, 
                T4.daytimewait, 
                T5.currentwait 
FROM   waits 
       left join (SELECT Avg(waittime) AS dayWait, 
                         ride 
                  FROM   waits 
                  WHERE  dayofweek = '" . $dow . "' 
                  GROUP  BY ride) AS T2 
              ON T2.ride = waits.ride 
       left join (SELECT Avg(waittime) AS timeWait, 
                         ride 
                  FROM   waits 
                  WHERE  currenttime >= '" . $minusTime . "' 
                         AND currenttime <= '" . $plusTime . "' 
                  GROUP  BY ride) AS T3 
              ON T3.ride = waits.ride 
       left join (SELECT Avg(waittime) AS dayTimeWait, 
                         ride 
                  FROM   waits 
                  WHERE  currenttime >= '" . $minusTime . "' 
                         AND currenttime <= '" . $plusTime . "' 
                         AND dayofweek = '" . $dow . "' 
                  GROUP  BY ride) AS T4 
              ON T4.ride = waits.ride 
       left join (SELECT waittime AS currentWait, 
                         ride 
                  FROM   waits 
                  GROUP  BY ride 
                  ORDER  BY wid DESC) AS T5 
              ON T5.ride = waits.ride 
WHERE  park = '" . getPark() . "'

http://pastebin.com/DDSpGFWQ

為了讓這個查詢的應用程序，我用它來填充這個鏈接公園的想法- http://www.wdwhelper.com 。 如果您需要其他信息，請告訴我。

我還在尋找有關子查詢T5的特定幫助-我想獲取每次騎行的最新（最后添加到數據庫）waitTime-任何想法這怎么可能？ 謝謝！

Answer 1

只需執行一個子查詢代替四個子查詢，然后讀取一次表即可，而不是四次，
您將節省3/4（75％）的時間：

SELECT ride , 
   Avg( CASE WHEN dayofweek = '" . $dow . "' THEN waittime END) AS dayWait,
   Avg( CASE WHEN currenttime >= '" . $minusTime . "' 
                  AND currenttime <= '" . $plusTime . "' 
        THEN waittime END) AS dayWait,  
   Avg( CASE WHEN currenttime >= '" . $minusTime . "' 
                  AND currenttime <= '" . $plusTime . "' 
                  AND dayofweek = '" . $dow . "' 
        THEN waittime END) AS dayWait,
    waittime AS currentWait                     
FROM   waits 
GROUP  BY ride