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  • MySQL必知必会(汇总数据, 聚集函数)

    SELECT AVG(prod_price) AS avg_price FROM products; #AVG只能用于单个列求平均值,如想计算多个列,必须用多个AVG()
    
    SELECT AVG(prod_price) AS avg_price FROM products
    WHERE vend_id = 1003; #AVG函数忽略列值为NULL的行
    
    SELECT COUNT(*) AS num_cust FROM customers; #此时不忽略列为NULL的行
    
    SELECT COUNT(cust_email) AS num_cust FROM customers; #此时忽略列为NULL的行
    
    SELECT MAX(prod_price) AS max_price    
    FROM products; #忽略列为NULL的行
    
    SELECT MIN(prod_price) AS max_price    
    FROM products; #忽略列为NULL的行
    
    SELECT SUM(quantity) AS items_ordered FROM orderitems
    WHERE order_num = 20005;
    
    SELECT SUM(item_price*quantity) AS total_price FROM orderitems
    WHERE order_num = 20005; #忽略列值为NULL的行
    
    SELECT AVG(DISTINCT prod_price) AS avg_price FROM products # 以上5个聚集函数,默认是对所有的行进行计算(ALL),
    WHERE vend_id = 1003; #如果只包含不同的值,可在列名前指定DISTINCT,但DISTINCT不能用于COUNT(*)。
    
    SELECT COUNT(*) AS num_items, MIN(prod_price) AS price_min, 
           MAX(prod_price) AS price_max, AVG(prod_price) AS price_avg FROM products;

    MySQL常用的聚集函数如下表,另外MySQL还支持一系列的标准偏差聚集函数:

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  • 原文地址:https://www.cnblogs.com/lasclocker/p/4750972.html
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