sqlite查询多对多表相关的两张表，一张表有重复

Question

我有 2 张桌子：

Attendee(AttendeeId integer PRIMARY KEY, LastName, FirstName, etc...)
Company(CompanyId integer PRIMARY KEY, CompanyName, etc ...)

以及它们之上的多对多关系：

CompanyAttendeeRelation
(
    CompanyId integer REFERENCES Company (CompanyId),
    AttendeeId integer REFERENCES Attendee (AttendeeId),
    PRIMARY KEY (CompanyId, AttendeeId)
);

因此，根据上述情况，一家公司可以有多个与会者，并且一名与会者可以在多个公司中。

但是，我已经尝试过，例如：

SELECT   (LastName || " " || FirstName) as AttendeeName,  
         CompanyAttendeeRelation.AttendeeId, 
         CompanyAttendeeRelation.CompanyId 

FROM     Attendee join CompanyAttendeeRelation on CompanyAttendeeRelation.AttendeeId = Attendee.AttendeeId join Company on CompanyAttendeeRelation.CompanyId = Company.CompanyId 

GROUP BY LastName;

但我没有让所有公司都参加。也就是说，例如，假设有AttendeeId = 15 的与会者有一个有CompanyId = 20 的公司和另一个有CompanyId = 100 的公司，我希望得到：

Doe John|15|20

Doe John|15|100

但我只得到：

Doe John|15|100

所以我不确定是否是 JOIN 在第一场比赛中停止，而第二场比赛永远不会被计算，或者是否某个地方重复（在本例中为 AttendeeId）被忽略。

有什么想法吗？

Answer 1

如果你想要所有的组合，你为什么要GROUP BY？不试一试。

SELECT (LastName || " " || FirstName) as AttendeeName,  
       CompanyAttendeeRelation.AttendeeId, 
       CompanyAttendeeRelation.CompanyId 

FROM   Attendee
JOIN   CompanyAttendeeRelation ON CompanyAttendeeRelation.AttendeeId = Attendee.AttendeeId
JOIN   Company on CompanyAttendeeRelation.CompanyId = Company.CompanyId; 
-- GROUP  BY LastName;

更好的是，对于这个特定的查询，您根本不需要加入 Company。所以试试这个：

SELECT (LastName || " " || FirstName) as AttendeeName,  
       CompanyAttendeeRelation.AttendeeId, 
       CompanyAttendeeRelation.CompanyId 

FROM   Attendee
JOIN   CompanyAttendeeRelation ON CompanyAttendeeRelation.AttendeeId = Attendee.AttendeeId;