第十二章：汇总数据

@author: Tobin
@date: 2019/10/28 22:32:57

聚集函数：运行在行组上，计算和返回单个值的函数。

# AVG()函数，忽略值为NULL的行
SELECT AVG(prod_price) AS avg_price
FROM products;

# COUNT()函数
# COUNT(*)，对行的数目进行计数，不管是否NULL
# COUNT(column)对特定列中具有值的行进行计数，忽略NULL值
SELECT COUNT(*) AS num_cust
FROM customers;

SELECT COUNT(cust_email) AS num_cust
FROM customers;

# MAX()函数和MIN()函数，对非数值数据也有用。忽略NULL。
# SUM()函数
SELECT SUM(item_price*quantity) AS total_price
FROM orderitems
WHERE order_num = 20005;

# DISTINCT，聚集不同值，只计算不同价格的平均值
SELECT AVG(DISTINCT prod_price) AS avg_price
FROM products
WHERE vend_id = 1003;

# 使用多个聚集函数
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;