如何使用mySQL内部连接3个表？

Question

I have joined two tables as below correctly

SELECT m.id as mid, c.id as cid FROM members m inner join companies c on m.id=c.id

and then I wanted to add a 3rd table also, but I can not get it working.

SELECT m.id as mid, c.id as cid, u.id as uid FROM members m inner join companies c on m.id=c.id inner join users u on m.id=u.id

What am I missing here?

Answer 1

Switch them from inner joins to left joins.

If you use the inner join, and a result does not exist in all three tables, it will be excluded, appearing to fail/not return results.

SELECT m.id as mid, c.id as cid, u.id as uid 
FROM members m 
left join companies c on m.id=c.id 
left join users u on m.id=u.id

That should take care of all the issues you are seeing.

Answer 2

You're sharing your "id" column in Members with both users and companies, was this your intention? Really your tables should have differing IDs and only join on foreign keys.

eg

Members.id = 12345 Members.CompanyFk = 45632

Companies.id = 45632

Join members.companyfk to companies.id