MySQL数据汇总与集聚函数MySQL系列（七）
MySQL数据汇总与聚集函数——MySQL系列（七） 1、聚集函数 聚集函数是运行在行组上，计算和返回单个的函数。 SQL聚集函数 函数 说明 AVG() 返回某列的平均 COUNT() 返回某列的行数 MAX() 返回某列的最大 MIN() 返回某列的最小 SUM() 返回某个列之和 （1）、AV
MySQL数据汇总与聚集函数——MySQL系列（七）1、聚集函数
聚集函数是运行在行组上，计算和返回单个值的函数。
| 函数 | 说明 |
|---|---|
| AVG() | 返回某列的平均值 |
| COUNT() | 返回某列的行数 |
| MAX() | 返回某列的最大值 |
| MIN() | 返回某列的最小值 |
| SUM() | 返回某个列之和 |
（1）、AVG()函数
可以返回所有列的平均值，也可以返回特定列的平均值。SELECT AVG(prd_price) AS avg_price FROM products Where vend_id=1003;
将过滤出vend_id=1003的产品，avg是这些产品的平均值.
（2）、COUNT()函数
COUNT(*)对表中行的数目进行计数，不管表列中包含的是空（NULL）还是非空值； COUNT(column)对特定列中有值的行进行计算，忽略NULL值。（3）、MAX()函数
MAX()返回指定列的最大值，要求指定列名，忽略NULL值。 在MySQL中，MAX()函数可以对非数据列使用，在用于文本数据时，如果数据按相应的列排序，MAX()返回最后一行。（4）、MIN()函数
MIN()返回指定列的最小值，要求指定列名，忽略NULL值。在MySQL中，MIN()函数可以对非数据列使用，在用于文本数据时，如果数据按相应的列排序，MIN()返回最前面的一行。
（5）、SUM()函数
用来返回指定列的和（总计），忽略NULL值的行。SELECT SUM(num) AS prod_sum---返回指定订单号中的商品数量 FROM products Where order_id=123456;
SELECT SUM(item_price*num) AS totol_price FROM order_items--返回订单中所有商品价格和。 Where order_id=123456;
2、聚集不同值
DISTINCT关键字 如下面的SQL将返回vend_id=1003的不同价格商品的平均值SELECT AVG(DISTINCT prd_price) AS avg_price FROM products Where vend_id=1003;
3、组合聚集函数
聚集函数可以组合使用SELECT count(*) AS num,
MIN(prod_price) AS price_min,
MAX(prod_price) AS price_max,
AVG(prod_price) AS price_avg
FROM products;
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