Question

I currently have this sql statement that I wrote and it works but it’s not quite what I want. I’ve been working on it for hours but can’t seem to get any further.

select parent.id as parent_id, parent.subject as parent,s.id,s.subject from (select s.id, s.subject from subjects s where parent_id = 0) parent join subjects s on parent.id = s.parent_id order by parent.subject, s.subject

It’s grabbing all the subjects and ordering correctly but I also want to return the parent subject (parent_id = 0) at the top of each grouping. This is because some parents may not have subjects underneath but I still need to return them. Also the ordering is off when I try to do it as I want the parent first then it’s child subjects. Hope that makes sense but if not just ask.

Any help would be appreciated.

Thanks

Steve

Answer 1

You’re talking about grouping sets of rows by their parent rows.

The only way I know how to do this in MySQL is using the GROUP_CONCAT() function which won’t group the subjects by row, but rather create a grouped string.

Here’s what you can do:

SELECT
    a.id,
    a.subject,
    GROUP_CONCAT(CONCAT(b.id, ':::', b.subject) ORDER BY b.subject SEPARATOR '|||') AS subjectlist
FROM
    subjects a
LEFT JOIN
    subjects b ON a.id = b.parent_id
WHERE
    a.parent_id = 0
GROUP BY
    a.id,
    a.subject
ORDER BY 
    a.subject

So this will give you a result set like:

id            |     subject     |     subjectlist
---------------------------------------------------------------------
2             |     subj1       |     23:::childsubj1|||28:::childsubj4
3             |     subj2       |     18:::childsubj8|||55:::childsubj16
4             |     subj3       |     NULL

Depending on what language you are using in your application, you may be able to “explode” the subjects string into arrays delimited first by ||| which separates each subject, then ::: which separates that subject’s ID and name.

Obviously, the downside of this is you have to make sure that your child subject name does not contain either ||| or ::: or whichever delimiters you decide to use.

Edit: Experimentally, I came up with this alternative solution which may be closer to what you’re looking for:

Try:

SELECT
    c.subj,
    c.id
FROM
    (
        SELECT
            CONCAT('---> ', b.subject) AS subj,
            b.id,
            CONCAT(a.subject, b.subject) AS orderfactor
        FROM
            subjects a
        INNER JOIN
            subjects b ON a.id = b.parent_id
        WHERE
            a.parent_id = 0

        UNION ALL

        SELECT
            subject AS subj, 
            id, 
            subject AS orderfactor
        FROM
            subjects
        WHERE 
            parent_id = 0
    ) c
ORDER BY
    c.orderfactor

This query should give you a result along the lines of:

subject                                 |       id       |
----------------------------------------------------------
subj1                                   |       2        |
---> childsubj1                         |       23       |
---> childsubj4                         |       28       |
subj2                                   |       3        |
---> childsubj8                         |       18       |
---> childsubj16                        |       55       |
subj3                                   |       4        |
subj4                                   |       5        |
---> childsubj10                        |       79       |