SQL加入和计算3个表，包括NULL [关闭]

Question

Link to relevant schema in question

对于 SQL。进行查询以显示 所有 已帮助客户的员工并显示他们的发票数量。我的问题是我可以显示员工人数和 ID，但我无法从查询中显示所有员工。

输出应该类似于

ID - Last - First - COUNT(invoices)

如果为 NULL，名称也应该显示出来。

我试过了：

SELECT e.EmployeeId,e.LastName, e.FirstName, COUNT() as NUM_INVOICE
FROM employees AS e
LEFT OUTER JOIN customers as c ON e.EmployeeId=c.SupportRepId
INNER JOIN invoices as i ON c.CustomerId =i.CustomerId
GROUP BY e.EmployeeId
ORDER BY NUM_INVOICE DESC

它给了：Output from my best attempt

问题是，这没有显示没有通过客户表和发票表连接的任何发票的剩余 id。它缺少雇员表中的 5 个名称，它们应该显示 NULL 作为计数。我认为左内连接可以解决这个问题并“带来”员工表中的名称，但它们被删除了。

Answer 1

问题在于，这没有显示剩余的 id 没有通过客户表和发票表连接的任何发票。

然后，使用LEFT JOIN：

SELECT 
    e.EmployeeId, e.LastName, e.FirstName, COUNT(i.CustomerId) AS NUM_INVOICE
FROM employees AS e
LEFT JOIN customers AS c ON e.EmployeeId = c.SupportRepId
LEFT JOIN invoices AS i ON c.CustomerId = i.CustomerId
GROUP BY e.EmployeeId
ORDER BY NUM_INVOICE DESC

Answer 2

只是为了展示另一种选择：我更喜欢在加入数据之前汇总数据。在这里，我会将发票计数加入员工：

select e.employeeid, e.lastname, e.firstname, coalesce(ic.cnt, 0) as num_invoice
from employees as e
left outer join
(
  select c.supportrepid, count(*) as cnt
  from customers as c 
  inner join invoices as i on i.customerid = c.customerid
  group by c.supportrepid
) ic on ic.supportrepid = e.employeeid
order by num_invoice desc;