選擇每個客戶的名字和時間，他們都在會議花費的總金額

Question

我有一個MySQL數據庫下表設置：

Client: 
id 
name 

Session: 
id 
service_id 
date 

Attendance: 
id 
client_id 
session_id 

Service: 
id 
duration 

客戶端有一個多對多的關係，通過考勤表會話。

我想編寫一個查詢將返回每個客戶的名字和他們在會議已經花的時間總量，這樣的：

Name  Total Time (Mins) 
Person A 200 
Person B 500 
Person C 100 

到目前爲止，我的查詢看起來是這樣的：

SELECT 
(SELECT SUM(services.duration) 
FROM services 
INNER JOIN sessions 
ON sessions.service_id = services.id 
INNER JOIN attendances 
ON attendances.session_id = session.id 
INNER JOIN clients 
ON clients.id = attendances.client_id 
GROUP BY clients.id) AS total_duration, 
client.name 
FROM clients 

但是，這不會返回任何結果。然而，如果我單獨運行查詢的任一部分，我得到我所需要的數據，即：

SELECT name FROM clients 

OR

SELECT SUM(services.duration) 
FROM services 
INNER JOIN sessions 
ON sessions.service_id = services.id 
INNER JOIN attendances 
ON attendances.session_id = session.id 
INNER JOIN clients 
ON clients.id = attendances.client_id 
GROUP BY clients.id 

有人能看到一個原因，當我結合它不會工作他們倆？

Answer 1

您在相關查詢中缺少WHERE條件。我重寫了只有連接的查詢以使其更易讀：

SELECT c.name, sum(srv.duration) 
FROM clients c 
    JOIN attendances a ON a.client_id = c.id 
    JOIN sessions s ON s.id - a.session_id 
    JOIN services srv ON srv.id = s.service_id 
GROUP BY c.name