SQL - 连接两个表时无法执行不同的计数

Question

我有两个表：Customers 和 Orders。我在 CustomerID 列上对客户和订单进行左连接，该列是客户的主键和订单的外键。

1. 当我在加入后列出 CustomerID 时，我得到了预期的列表。
2. 当我计算 CustomerID 的数量时，我再次得到了我期望的记录数。
3. 当我对 CustomerID 使用不同计数时，我得到一个错误。

1.

select  Customers.CustomerID as list
from Customers left join Orders on Customers.CustomerID = Orders.CustomerID
where Customers.CustomerID = 4;

2.

select  count(Customers.CustomerID) as numRecord
from Customers left join Orders on Customers.CustomerID = Orders.CustomerID
where Customers.CustomerID = 4;

3.

select  count(distinct (Customers.CustomerID)) as numRecord
from Customers left join Orders on Customers.CustomerID = Orders.CustomerID
where Customers.CustomerID = 4;

我不明白错误在哪里。任何帮助，将不胜感激。 错误：

Error in SQL:
Syntax error (missing operator) in query expression 'count(distinct Customers.CustomerID)'.

Answer 1

您不需要在count(distinct ..)中的Customers.CustomerID周围使用()

select  count(distinct Customers.CustomerID) as numRecord
from Customers 
left join Orders on Customers.CustomerID = Orders.CustomerID
where Customers.CustomerID = 4;