分组数据

1 介绍

分组返回数据，不是 where 的那种返回特定数据

2 创建分组 (group by)

select vend_id, count(*) as num_prods from products group by vend_id;

2.1 分析

按 vend_id 字段分组，然后统计总数返回数据

3 过滤分组 (having)

select cust_id, count(*) as orders 
from orders
group by cust_id
having count(*) >= 2;

3.1 分析

筛选出需要数据

3.2 where 和 having 区别

where 过滤行，having 过滤分组