MySql Impass - can't move forward - mysql

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 |