Question

I have 3 tables, ‘u’ ‘d’ ‘s’

‘u’ has

• userid
• divid

‘d’ has

• divid
• divname

‘s’ has

• sname
• primaryuserid
• secondaryuserid

Now what I’d like to do is display a table with rows of the following format

• userid, divname, sname

Plus figure out a way to decipher whether userid is a primary or secondary for this sname table.

I’m able to show userid and divname using a left join, but I don’t know how I would add a third table? To make it trickier, there can be more than 1 snames for each userid, up to ~20. Is there a way to display 0-20 snames depending on the userid, separated with commas?

What I have currently is just and u and d tables corresponding to each other.

SELECT
    e.userid,
    e.divid, 
    d.divname 
FROM 
    e
LEFT JOIN d ON (e.divid = d.id)
ORDER BY e.userid

Answer 1

You’re looking for a couple of different things here. First of all, the “way to display 0-20 snames depending on the userid, separated with commas” can be done with MySQL’s GROUP_CONCAT() function, so your query would look like this (no distinction between primary and secondary yet):

SELECT u.userid,
    d.divname,
    GROUP_CONCAT(s.sname SEPARATOR ', ') AS "snames"
FROM u
    LEFT JOIN d ON d.divid = u.divid
    LEFT JOIN s ON (s.primaryuserid = u.userid OR s.secondaryuserid = u.userid);

Now, to be able to distinguish whether the user is primary or secondary, you have to get a little fancier. I’d probably do it with a union, like this:

SELECT u.userid,
    d.divname,
    GROUP_CONCAT(s.sname SEPARATOR ', ') AS "snames",
    'Primary' AS "category"
FROM u
    LEFT JOIN d ON d.divid = u.divid
    LEFT JOIN s ON s.primaryuserid = u.userid
UNION ALL
SELECT u.userid,
    d.divname,
    GROUP_CONCAT(s.sname SEPARATOR ', ') AS "snames",
    'Secondary' AS "category"
FROM u
    LEFT JOIN d ON d.divid = u.divid
    LEFT JOIN s ON s.secondaryuserid = u.userid

This will give you two rows (one Primary, one Secondary) for each userid with comma-separated lists of the snames.