创建分组
select vend_id, count(*) as num_prods
from products
group by vend_id;
group by 语句的规定：
- 可以包含任意数目的列，因而可以对分组进行嵌套
- 必须出现在where语句之后，having语句之前
等等
过滤分组
过滤掉不符合条件的分组，使用having而不是where
** having和where的区别 **：
** where在数据分组前进行过滤，having在数据分组后进行过滤，where过滤的是行，having过滤的是分组 **
select cust_id, count(*) as orders
from orders
group by cust_id
having count(*) >= 2;
select vend_id, count(*) as num_prods
from products
where prod_price >= 4
group by vend_id
having count(*) >= 2;
分组和排序
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select order_num,count(*) as items
from orderItems
group by order_num
having count(*) >= 3
order by items, order_num;
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