SQL - 在连接两个表时无法执行不同的计数

Question

我有两个表：客户和订单。我在CustomerID列上进行了客户和订单的左连接，这是客户中的主键和订单中的外键。

1. 当我在加入后列出CustomerID时，我按预期获得列表。
2. 当我计算CustomerID的数量时，我再次得到我期待的记录数。
3. 当我对CustomerID使用非重复计数时，我收到错误。

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;

我无法理解错误在哪里。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count（distinct ..）

中的Customers.CustomerID周围的（）

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