Question

我有两张桌子：

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

和他们之间的多对多关系：

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

因此，从上面看，一家公司可以有一个以上的与会者，一个与会者可以在一个以上的公司。

但是，我试过了，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;

但我并没有让所有公司参加。也就是说，例如，有AttendeeId = 15的与会者有CompanyId = 20的公司和CompanyId = 100的公司，我希望得到：

Doe John|15|20

Doe John|15|100

但我只是得到了：

Doe John|15|100

所以我不确定它是第一次匹配时停止的JOIN，第二次永远不会被计算出来，或者某个重复的地方（在这种情况下为AttendeeId）被忽略了。

有什么想法吗？

Answer 1

如果您想要所有组合，为什么要GROUP BY？不用试试吧。

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，对于此特定查询，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;

sqlite查询由多对多表关联的两个表，在一个表上有重复项

1 个答案: