mysql笔记(10)

Posted 2021-07-18 HardyDragon_CC

使用 GROUP BY 分组数据，从输出中可以看到，供应商1001有3个产品，供应商1002有2个产品，供应商1003有7个产品，而供应商1005有2个产品；注意不要和 order by 弄混了。

SELECT vend_id ,count(*) AS num_prods 
FROM products p 
GROUP BY vend_id ;

使用WITH ROLLUP关键字，可以得到每个分组以及每个分组汇总级别（针对每个分组）的值

SELECT vend_id ,count(*) AS num_prods 
FROM products p 
GROUP BY vend_id WITH  ROLLUP ;

使用 having 过滤分组，不是 where 。HAVING支持所有WHERE操作符。

SELECT cust_id ,count(*) AS orders 
FROM orders o 
GROUP BY cust_id ; 

使用 having 过滤出 数量大于等于 2的分组。

SELECT cust_id ,count(*) AS orders 
FROM orders o 
GROUP BY cust_id 
HAVING count(*)>=2; 

HAVING和WHERE的差别 这里有另一种理解方法，WHERE在数据分组前进行过滤，HAVING在数据分组后进行过滤

下面列出具有2个（含）以上、价格为10（含）以上的产品的供应商；

• WHERE子句过滤所有prod_price至少为10的行。然后按vend_id分组数据，HAVING子句过滤计数为2或2以上的分组。

• 如果没有WHERE子句，将会多检索出两行（供应商1002，销售的所有产品价格都在10以下；供应商1001，销售3个产品，但只有一个产品的价格大于等于10）：

SELECT vend_id ,count(*) AS num_prods 
FROM products p 
WHERE prod_price >= 10
GROUP BY vend_id 
HAVING count(*) >= 2; 

SELECT vend_id ,count(*) AS num_prods 
FROM products p 
GROUP BY vend_id 
HAVING count(*) >= 2; 

SELECT order_num ,sum(quantity*item_price) AS ordertotal 
FROM orderitems o 
GROUP BY order_num 
HAVING sum(quantity*item_price) >= 50; 

可以使用 order by 利用 AS 生成的列排序；

SELECT order_num ,sum(quantity*item_price) AS ordertotal 
FROM orderitems o 
GROUP BY order_num 
HAVING sum(quantity*item_price) >= 50
ORDER BY ordertotal;

SELECT order_num ,sum(quantity*item_price) AS ordertotal 
FROM orderitems o 
GROUP BY order_num 
HAVING sum(quantity*item_price) >= 50
ORDER BY  ordertotal DESC;

总结： 使用 group by 分组数据，使用 having 过滤分组，可以配合 where 和 其他函数使用。