mysql - 使用SQL计算平均客户数

Question

数据结构请见上文。我正在尝试编写一个 SQL 查询来获取每个 session 的平均客户数

我的尝试:

select avg(A.NumberCustomer)
from(
select SessionName, count(distinct customers.Idcustomer) as NumberCustomer, 
from customers, enrollments, sessions
where customers.Idcustomer=enrollments.Idcustomer and enrollments.Idsession=sessions.Idsession
group by sessions.SessionName
) A

但我似乎在 from customers, enrollments, sessions 行中收到错误对此不确定，感谢任何帮助。

谢谢

Answer 1

你有多余的逗号，你应该删除:

select avg(A.NumberCustomer)
from(
select SessionName, 
       count(distinct customers.Idcustomer) as NumberCustomer, #<--- here
from customers, enrollments, sessions
where customers.Idcustomer=enrollments.Idcustomer 
  and enrollments.Idsession=sessions.Idsession
group by sessions.SessionName
) A

顺便说一句，出于可读性原因，我建议您改用 SQL'99 连接语法:

SELECT 
    avg(A.NumberCustomer)
FROM (
   select 
      SessionName, 
      count(distinct customers.Idcustomer) as NumberCustomer
   from customers 
   inner join enrollments
      on customers.Idcustomer=enrollments.Idcustomer 
   inner join sessions
      on enrollments.Idsession=sessions.Idsession
   group by sessions.SessionName
) A

此外，关于问题的图表很好，下次记得包含您的错误消息。