Question

-Goal-

I would like to fetch users who do not have an entry within the ‘bar’ table. These users will also belong to a certain ‘usergroup’ and have restrictions within the ‘user’ table itself.

Simplified table structure

User

userid - auto-increment    
username - string    
userage - integer    
usergroup_usergroupid - foreign key

usergroup

usergroup - auto-increment    
usergroupname - string

user_has_bar

user_userid - foreign key    
bar_barid - foreign key

-Query…so far-

SELECT username     
FROM user as u, usergroup as ug, user_has_bar as ub    
WHERE    
u.usergroup_usergroupid = ug.usergroupid AND    
u.userage = 10 AND  <-- lets pretend this exists    
u.location_locationid = 1 AND <-- Same here     
u.userid ... ub.user_userid;

-Headache-

It is this last line that has me throwing up blanks. I believe from what I have read up on, the way to perform this operation is with a LEFT JOIN. However, everything i have read so far ends up being quite confusing.

If possible, I would like to keep this operation within a mySQL query and then optimise later.It is created by a PHP mySQL generator I coded and I would like to understand the simple syntax and then dig more in to it. Any approach is VERY welcome though

Thank you kindly for reading this question.

Answer 1

SELECT username
    FROM user u
        INNER JOIN usergroup ug
            ON u.usergroup_usergroupid = ug.usergroupid 
        LEFT JOIN user_has_bar ub
            ON u.userid = ub.user_userid
    WHERE u.userage = 10
        AND u.location_locationid = 1
        AND ub.user_userid IS NULL