Question

我试图显示客户从'Tucson'购买的产品的平均价格，但是这个查询返回null，即使有两个客户已经从Tuscon下订单。

select AVG(product_price) from product where product_id in
(select product_id from orderline where order_id in
(select order_id from ordertable where cust_id in
(Select cust_id from customer where city = 'Tuscon')))

Answer 1

SELECT AVG(p.product_price) FROM ( SELECT l.product_id FROM orderline l JOIN ordertable o ON o.order_id = l.order_id JOIN customer c ON c.cust_id = o.cust_id WHERE c.city = 'Tuscon' GROUP BY l.product_id ) q JOIN product p ON p.product_id = l.product_id 不是可识别的关键字。 MySQL解析器不知道如何处理它，因此它会引发关于＆＃34;无效语法的错误＆＃34;。

考虑使用连接操作代替嵌套的IN子查询。如果我们得到保证：

product_id在产品表中是唯一的
order_id在ordertable table中是唯一的
cust_id在客户表中是唯一的

然后我们可以得到一个等价的结果集，即订购的不同产品的平均价格......

SELECT AVG(p.product_price)
  FROM product p
  JOIN orderline l
    ON l.product_id = p.product_id
  JOIN ordertable o
    ON o.order_id = l.order_id
  JOIN customer c
    ON c.cust_id = o.cust_id
 WHERE c.city = 'Tuscon'

如果我们想要＆＃34;平均价格＆＃34;订购的所有产品（不同的结果，平均值考虑到产品订购的次数......然后我们可以使用这样的查询：

{{1}}

Answer 2

您在查询中使用SELECT i FROM ( select i0.i +i1.i*2 +i2.i*4 +i3.i*8 +i4.i*16 +i5.i*32 +i6.i*64 +i7.i*128 +i8.i*256 +i9.i*512 as i from (select 0 as i union select 1) as i0 cross join (select 0 as i union select 1) as i1 cross join (select 0 as i union select 1) as i2 cross join (select 0 as i union select 1) as i3 cross join (select 0 as i union select 1) as i4 cross join (select 0 as i union select 1) as i5 cross join (select 0 as i union select 1) as i6 cross join (select 0 as i union select 1) as i7 cross join (select 0 as i union select 1) as i8 cross join (select 0 as i union select 1) as i9 ) Z WHERE i>=1 AND i<=1000;代替fom：from

这应该是select order_id fom ordertable where cust_id in

错误1064 - 在声明中

2 个答案: