聚集函數以及如何利用聚集函數匯總數據

聚集函數運行在行組上，返回單個函數值

SQL聚集函數

AVG()函數：?? 只能確定特定數列之的平均值，并且會忽略NULL值

select avg(prod_price) as avg_price from products;

select prod_price,prod_name,prod_id from products;

select sum(prod_price) as sum_price from products;

select prod_price,prod_name,prod_id from products where vend_id =1003;

select avg(prod_price) as avg_price from products where vend_id =1003;

select sum(prod_price) as sum_price from products where vend_id =1003;

當使用聚集函數時，不能進行篩選操作，會報錯。因為聚集函數返回的時單個的函數值，而篩選操作返回的時一系列行

COUNT()函數：? 計數函數

select count(*) as num_cust from customers;

select cust_name from customers;

select count(cust_email) as num_cust from customers;

select cust_email from customers;

這里對cust_email進行計數，count()函數會忽略null值，如果是count(*)則不會忽略NULL值

MAX（）函數：?? 返回指定列最大值? 忽略NULL值

MIN（）函數：功能同上

select max(prod_price) as max_price from products;

select prod_price from products;

sum()函數：? 求和

select sum(quantity) as items_ordered from orderitems where order_num = 20005;

select quantity from orderitems where order_num = 20005;

select item_price,quantity from orderitems where order_num = 20005;

select sum(quantity*item_price) as total_money from orderitems where order_num = 20005;

聚集不同值

select avg(distinct prod_price) as avg_price from products where vend_id = 1003;

指定供貨商1003，只考慮不同價格distinct prod_price