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 建立分组(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，而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

筛选出须要数据

3.2 where和having 区别

where过滤行，having过滤分组