MySQL--分組數據

1、數據分組

#連接數據庫
use newschema;
#查看表中數據
select *from products；
#返回供應商1003提供的產品數目
select count(*) as num_prods from products where vend_id=1003;

2、創建分組

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

**Group By 子句必須出現在where自居之后，order by 子句之前。

#使用with rollup
select vend_id,count(*) as num_prods from products group by vend_id with rollup;

#使用with rollup關鍵字，可以得到每個分組以及每個分組匯總級別（針對每個分組）的值。

 

3、過濾分組

所有類型的where子句都可以用having來替代。唯一差別師where過濾行，而having過濾分組。

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

having和where的差別：where在數據分組前進行過濾，having在數據分組后進行過濾。

select vend_id,count(*) as num_prods 
from products 
where prod_price>=10 
group by vend_id 
having count(*)>=2;

分析：where子句過濾所有prod_price至少為10 的行，然后按照cend_id 分組，having子句過濾計數為2或2以上的分組。

 

select vend_id,count(*) as num_prods
 from products 
group by vend_id 
having count(*)>=2;

 4、分組和排序

select order_num,sum(quantity*item_price) as ordertotal 
from orderitems 
group by order_num 
having sum(quantity*item_price)>=50;

 

select order_num,sum(quantity*item_price) as ordertotal 
from orderitems 
group by order_num 
having sum(quantity*item_price)>=50 
order by ordertotal;

 

5、select子句順序

下列表是在使用select語句時必須遵循的次序

select

from

where

group by

having

order by

limit