本文为本人学习书籍《MySQL必知必会》笔记系列,欢迎持续关注,有问题随时留言评论,一起探讨学习~
12.汇总数据
12.1 聚集函数
聚集函数:运行在行组上,计算和返回单个值的函数。 AVG/COUNT/MAX/MIN/SUN():返回某列的平均值/行数/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()函数有两种使用方式。
- COUNT(*)对表中行的数目进行计数,不管表列中包含的是空值(NULL)还是非空。
- COUNT(column)对column列计数,忽略空值
SELECT COUNT(*) AS num_Name,
COUNT(Description) AS num_Description
FROM product;
12.1.3 MAX/MIN/SUN()函数
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;