如何联接或左联接三个表以从其中一个表中获取数据？

Question

I have a users table, a jobs table, and a files table; the exact table names are user_table , jobs , and files .

The files table and jobs table have a common field, and the jobs table and users table have a common field, but all 3 do not have a common field. So files table has jobid which equals id from jobs table, and jobs table has userid which equals userid from users table.

Example records

files table

id    name        path                jobid
-------------------------------------------
7     test.gif    uploads/test.gif    130

jobs table

id     userid    ponumber    date
------------------------------------------------
130    1013      2322        10/14/2013 2:55:1pm

user_table table

userid    username    email              fname    sname
-------------------------------------------------------
1013      username    email@email.com    first    last

File 7 belongs to job 130, and job 130 belongs to user 1013. So when displaying files on a page, I want to also display the owner of the file, but that's two tables away.

How can I do this? I haven't tried anything because I am new to MySQL programming. If I were to try and guess something, it might go something like this:

SELECT *
FROM user_table u
INNER JOIN jobs j USING (userid)
LEFT JOIN files f ON j.id = f.jobid

which probably is such a wild guess, it's not even worth mentioning.

Answer 1

Well, if all users have jobs and all jobs have files OR you want to only show users with jobs that have files then

SELECT *
FROM user_table u
INNER JOIN jobs j ON j.id=u.userID
INNER JOIN files f ON j.id = f.jobid

will do that.

If all users DON'T have jobs and all jobs DON'T have files OR you want to show all users regardless if they have a job with files or not then

SELECT *
FROM user_table AS u
LEFT JOIN jobs AS j ON j.id=u.userID
LEFT JOIN files AS f ON j.id = f.jobid

will do that.

HERE goes a description of joins that might help you understand joins a bit better.

Answer 2

SELECT user_table.username, files.*
FROM files 
INNER JOIN jobs ON files.jobid = jobs.id
INNER JOIN user_table ON jobs.userid = user_table.userid

Answer 3

SELECT files.id, files.name, jobs.id, user_table.username
FROM files LEFT JOIN jobs
ON files.id = jobs.id LEFT JOIN user_table
ON jobs.userid = user_table.userid