子查詢還是不寫這個查詢？

Question

我的SQL查詢：

select 
    *, 
    (select sum(Amount) as TotalAmount 
    from Payment 
    where Customer.Id = Payment.CustomerId) 
from Customer 

結果：

Id - CustomerName - CustomerPhoneNumber - TotalAmount 
1 - TestUser  - TestLastName  - 3000 
2 - TestUser2 - TestLastName2  - 2500 

---

這是工作如我所料。但是有沒有更好的方法來獲得相同的結果？

Answer 1

如果它是MySQL的，那麼你就可以做到這一點...

SELECT 
    Customer.*, 
    SUM(Payment.Amount) AS TotalAmount 
FROM 
    Customer 
LEFT JOIN 
    Payment 
    ON Payment.CustomerId = Customer.Id 
GROUP BY 
    Customer.id 

在其他品種的SQL，則需要GROUP BY一切，是選擇，但不是彙總。

SELECT 
    Customer.Id, 
    Customer.Name, 
    Customer.PhoneNumber, 
  SUM(Payment.Amount) AS TotalAmount 
FROM 
  Customer 
LEFT JOIN 
  Payment 
    ON Payment.CustomerId = Customer.Id 
GROUP BY 
  Customer.Id, 
    Customer.Name, 
    Customer.PhoneNumber 

Answer 2

您可以LEFT JOIN和GROUP BY重寫它：

select 
a.*, sum(b.Amount) as TotalAmount  
from Customer a 
left join Payment on (a.Id = b.CustomerId) 
GROUP BY a.Id -- for mysql only, for other RDMS you have to list all Customer fields 

或者

select 
a.*, b.TotalAmount  
from Customer a 
left join 
(SELECT CustomerId, SUM(Amount) as TotalAmount 
FROM Payment GROUP BY CustomerId)b ON (b.CustomerId = a.Id)