分組數據

1 介紹

分組返回數據，不是where的那種返回特定數據

2 建立分組(group by)

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

2.1 分析

按vend_id字段分組，而後統計總數返回數據

3 過濾分組(having)

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

3.1 分析

篩選出須要數據

3.2 where和having 區別

where過濾行，having過濾分組