在電影數據庫上極慢的mySQL查詢

Question

我有一個php文件查詢mySql數據庫設置與一個表上的電影和另一個movieRatings。我想要統計一部特定電影的「五星級」，「四星級」等評級數量。我的查詢很有效，但是速度非常慢，因爲我意識到它會通過整個movieRatings表格5次查看我想要查看的每部電影。

我的查詢：

$sql = "SELECT * 
       FROM MOVIE_INFO, 
       (SELECT count(ratingValue) as star5 FROM MOVIE_RATINGS2 WHERE itemID=$b[0] AND ratingValue = 5) SUB, 
       (SELECT count(ratingValue) as star4 FROM MOVIE_RATINGS2 WHERE itemID=$b[0] AND ratingValue = 4) SUB1, 
       (SELECT count(ratingValue) as star3 FROM MOVIE_RATINGS2 WHERE itemID=$b[0] AND ratingValue = 3) SUB2, 
       (SELECT count(ratingValue) as star2 FROM MOVIE_RATINGS2 WHERE itemID=$b[0] AND ratingValue = 2) SUB3, 
       (SELECT count(ratingValue) as star1 FROM MOVIE_RATINGS2 WHERE itemID=$b[0] AND ratingValue = 1) SUB4 

       WHERE movieID = $b[0]"; 

我想如果我可以從該特定影片的影片收視率一度拉子集，它會加速它的巨大的，但我不記得如何做到這一點。

Answer 1

您正在執行多個子選擇。相反，嘗試分組查詢，例如

SELECT ratingValue, COUNT(ratingValue) FROM MOVIE_INFO 
WHERE movieID = $b[0] 
GROUP BY ratingValue 

你會得到每星級的一行。第一列將是評級本身（例如1），第二列將是具有該星號數量的評級數目。