Question

因此，我有一个用户表和一个订单表。订单表通过user_id列链接到用户表。我想做的是运行一个查询，以获取所有用户以及他们下的订单数量和订单总成本。这就是我所拥有的：

select count(orders.id) as order_count, sum(orders.total_cost) as total_spent, CONCAT_WS(", ", `last_name`, `first_name`) AS `customer_name`, users.id as user_id
from users
left join orders on users.id = orders.user_id
where orders.status != 'Canceled'
group by user_id
order by order_count asc

我的问题是，它不会返回在订单表中没有记录的用户。理想情况下，我想查看所有客户并为未下订单的用户显示0（即使为null也可以）。任何帮助将不胜感激。

Answer 1

您快到了，只需要添加group by语句

select 
   count(orders.id) as order_count, 
   sum(orders.total_cost) as total_spent,
   CONCAT_WS(", ", `last_name`, `first_name`) AS `customer_name`, 
   users.id as user_id
from users
left join orders on users.id = orders.user_id and orders.status != 'Canceled'
group by  `customer_name`, user_id

还请注意，如果进行左联接，并且需要在左联接表上添加约束，则需要将其放在on子句中。如果将左侧条件放在where子句中，则左联接将隐式转换为内部联接

Answer 2

这是因为您有一个左联接，但是将左表放在了进行内部联接的where条件中。

这两个修复程序将条件放入了连接中：

select count(orders.id) as order_count, sum(orders.total_cost) as total_spent, CONCAT_WS(", ", `last_name`, `first_name`) AS `customer_name`, users.id as user_id
from users
left join orders on users.id = orders.user_id and orders.status != 'Canceled'
group by user_id
order by order_count asc

或测试是否为空

select count(orders.id) as order_count, sum(orders.total_cost) as total_spent, CONCAT_WS(", ", `last_name`, `first_name`) AS `customer_name`, users.id as user_id
from users
left join orders on users.id = orders.user_id
where coalesce(orders.status,'') != 'Canceled'
group by user_id
order by order_count asc

无法查询零记录的联接

2 个答案: