Question

I have an application that uses a nested set model class to organise my data, however I’m trying to write a query that will group_concat my results. I know I need to put some sub select statements somewhere but I can’t figure it out!

Here’s my structure at the moment:

table: person

 -----------+------------+-----------
|Person_ID  | Name       | Age       |
 -----------+------------+-----------
| 1         | Mark Vance | 19        |
| 2         | Michael Tsu| 22        |
| 3         | Mark Jones | 29        |
| 4         | Sara Young | 25        |
 -----------+------------+-----------

table: person_to_group

 ----+------------+-----------
|ID  | Person_ID  | Group_ID  |
 ----+------------+-----------
| 1  | 3          | 1         |
| 2  | 3          | 2         |
| 3  | 1          | 2         |
| 4  | 4          | 3         |
 ----+------------+-----------

table: groups

 ----------+--------------+--------------+-------------
|Group_ID  | Group_Name   | Group_Left   | Group_Right |
 ----------+--------------+--------------+-------------
| 1        | Root         | 1            | 6           |
| 2        | Node         | 2            | 5           |
| 3        | Sub Node     | 3            | 4           |
 ----------+--------------+--------------+-------------

I need to render something like this with my results:

//Grab the group_IDs for this person and put them in the class tag…

<li class="2 3">Sara Young is in the Sub Node Group</li>

Notice that although Sara is in the Sub Node group, she is still being given the id for Node aswell because she is a child of Node.

The following is the query that I am working with as a starting point.

SELECT *, GROUP_CONCAT( CAST( gg.Group_ID AS CHAR ) SEPARATOR ' ' ) Group_IDs
        FROM groups gg
        LEFT JOIN person_to_group AS t1 ON gg.Group_ID = t1.Group_ID
        LEFT JOIN person AS t2 ON t2.Person_ID = t1.Person_ID
        GROUP BY t2.per_ID 
        ORDER BY t2.Name ASC

Any help would be much appreciated!

Answer 1

Here’s how I’d write the query:

SELECT p.Name, 
  GROUP_CONCAT( g.Group_Name ) AS Group_List,
  GROUP_CONCAT( CAST( gg.Group_ID AS CHAR ) SEPARATOR ' ' ) AS Group_ID_List
FROM person AS p
INNER JOIN person_to_group AS pg ON p.Person_ID = pg.Person_ID
INNER JOIN groups AS g ON pg.Group_ID = g.Group_ID
INNER JOIN groups AS gg ON g.Group_Left BETWEEN gg.Group_Left AND gg.Group_Right
GROUP BY p.Name
ORDER BY p.Name ASC

Note that if you group by person name, you also need to GROUP_CONCAT the list of group names. According to your schema, a person could belong to multiple groups, because of the many-to-many relationship.

I also recommend against using SELECT * in general. Just specify the columns you need.