In what order are MySQL JOINs evaluated?

Question

I have the following query:

SELECT c.*
FROM companies AS c
JOIN users AS u USING(companyid)
JOIN jobs AS j USING(userid)
JOIN useraccounts AS us USING(userid         


        

Answer 1

1) Using is not exactly the same as on, but it is short hand where both tables have a column with the same name you are joining on... see: http://www.java2s.com/Tutorial/MySQL/0100__Table-Join/ThekeywordUSINGcanbeusedasareplacementfortheONkeywordduringthetableJoins.htm

It is more difficult to read in my opinion, so I'd go spelling out the joins.

3) It is not clear from this query, but I would guess it does not.

2) Assuming you are joining through the other tables (not all directly on companyies) the order in this query does matter... see comparisons below:

Origional:

SELECT c.* 
    FROM companies AS c 
    JOIN users AS u USING(companyid) 
    JOIN jobs AS j USING(userid) 
    JOIN useraccounts AS us USING(userid) 
WHERE j.jobid = 123

What I think it is likely suggesting:

SELECT c.* 
    FROM companies AS c 
    JOIN users AS u on u.companyid = c.companyid
    JOIN jobs AS j on j.userid = u.userid
    JOIN useraccounts AS us on us.userid = u.userid 
WHERE j.jobid = 123

You could switch you lines joining jobs & usersaccounts here.

What it would look like if everything joined on company:

SELECT c.* 
    FROM companies AS c 
    JOIN users AS u on u.companyid = c.companyid
    JOIN jobs AS j on j.userid = c.userid
    JOIN useraccounts AS us on us.userid = c.userid
WHERE j.jobid = 123

This doesn't really make logical sense... unless each user has their own company.

4.) The magic of sql is that you can only show certain columns but all of them are their for sorting and filtering...

if you returned

SELECT c.*, j.jobid....  

you could clearly see what it was filtering on, but the database server doesn't care if you output a row or not for filtering.