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 count the total amount of people in each group.

table: person_to_group

 ----+------------+-----------
|ID  | Person_ID  | Group_ID  |
 ----+------------+-----------
| 1  | 3          | 1         |
| 2  | 3          | 2         |
| 3  | 5          | 2         |
| 4  | 7          | 3         |
 ----+------------+-----------

table: groups

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

My query will be run within a while loop which lists all the group names.

I’m trying to accomplish a result like this:

Root – Members (3) <——- Notice that I want subgroups to be included in the count, and I don’t want members to be counted more than once.

Any help would be much appreciated!

Answer 1

Since you run it inside a while loop with group names you can probably get the group’s left and right values instead and put them into this query:

SELECT count(*) AS members FROM 
( SELECT DISTINCT ptg.person_ID FROM groups g 
JOIN person_to_group ptg
ON ptg.Group_ID=g.Group_ID
WHERE g.Group_Left  >= 1 
AND   g.Group_Right <= 6 ) m

This works for a given group. If you wanted to get a full list of groups with member count for each in one query you would have to use something like:

SELECT m.Group_Name, count(*) AS members FROM
  ( SELECT g.Group_ID, g.Group_Name, ptg.Person_ID 
    FROM groups g 
    JOIN groups gsub 
      ON gsub.Group_Left >= g.Group_Left AND gsub.Group_Right <= g.Group_Right
    JOIN person_to_group ptg
      ON gsub.Group_ID = ptg.Group_ID
    GROUP BY g.Group_ID, ptg.Person_ID ) m
GROUP BY m.Group_ID

However I think the first solution with a loop outside of sql would be more efficient.