MySQL學習筆記：彙總數據

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12.彙總數據

12.1 聚集函數

聚集函數：運行在行組上，計算和返回單個值的函數。 AVG/COUNT/MAX/MIN/SUN():返回某列的平均值/行數/最大值/最小值/某列之和

12.1.1 AVG():返回某列的平均值

SELECT AVG(Price1) AS Ave_Price1,
       AVG(Price2) AS Ave_Price2,
			 AVG(Price3) AS Ave_Price3,
			 AVG(Price4) AS Ave_Price4
FROM product
WHERE Price1<2000;

12.1.2 COUNT()函數有兩種使用方式。

1. COUNT(*)對錶中行的數目進行計數，不管表列中包含的是空值(NULL)還是非空。
2. COUNT(column)對column列計數，忽略空值

SELECT COUNT(*) AS num_Name,
       COUNT(Description) AS num_Description
FROM product;

12.1.3 MAX/MIN/SUN()函數

SELECT MAX(Price1) AS Max_Price1,
       MIN(Price1) AS Min_Price1,
			 SUM(SettleType) AS Sum1,
			 SUM(Price1*SettleType) AS Sum2
FROM product;

12.2 聚集不同值：DISTINCT()

SELECT AVG(DISTINCT(Price1)) AS AVG_DIS_Price1,
COUNT(DISTINCT(Name)) AS COUNT_DIS_Name
FROM product;