选择同一列两次+ JOIN

Question

我有一个系统，用户可以报告另一个用户。

我想获得以下数据

报告编号|举报动机|报告owner_id |报告所有者用户名|报告用户ID |报告用户用户名|创建日期

报告

id SERIAL PRIMARY KEY, 
owner_id INTEGER REFERENCES Registered(user_id), 
creation_date TIMESTAMP NOT NULL, 
motive VARCHAR(50) DEFAULT 'TOA Violation' NOT NULL 
active BOOLEAN DEFAULT TRUE 

Reporteduser

report_id INTEGER REFERENCES Report(id), 
user_id INTEGER REFERENCES Registered(user_id), 
PRIMARY KEY(report_id, user_id) 

人

id SERIAL PRIMARY KEY, 
username VARCHAR(20) UNIQUE NOT NULL, 
password VARCHAR(50) NOT NULL, 
email VARCHAR(50) UNIQUE NOT NULL 
creation_date TIMESTAMP NOT NULL 

我写

SELECT DISTINCT report.id, report.owner_id AS source_id, reporteduser.user_id AS target_id, report.motive, report.creation_date, 
(SELECT person.username 
FROM person 
JOIN reporteduser 
ON person.id = reporteduser.user_id) AS target_user, 
(SELECT person.username 
FROM person 
JOIN report 
ON person.id = report.owner_id) AS source_user 
FROM report 
JOIN person 
ON report.owner_id = person.id 
JOIN reporteduser 
ON reporteduser.report_id = report.id 
WHERE report.active = TRUE; 

问题是子查询一次给我所有的用户。它只有一个报告就能创造奇迹。

Answer 1

你并不需要所有的子查询，尝试这样的事情：

select r.*, o.name as owner_name, p.name as reported_name 
from report r 
join person o on o.id = r.owner_id 
join reporteduser ru on ru.report_id = r.id 
join person p on p.id = ru.user_id 
where r.active = true