Question

I need to have a 5 levels hierarchy for the users registered to a website. Every user is invited by another, and I need to know all descendants for a user. And also ancestors for a user.

I have in mind 2 solution.

1. Keeping a table with relationships this way. A closure table:

    ancestor_id  descendant_id  distance
    1            1              0
    2            2              0
    3            3              0
    4            4              0
    5            5              0
    6            6              0
    2            3              1

1. Having this table for relationships. Keeping in a table 5 levels ancestors. A “ancestors” table:

   user_id ancestor_level1_id ancestor_level2_id ancestor_level3_id ancestor_level4_id ancestor_level5_id
   10      9                  7                  4                  3                  2
   9       7                  4                  3                  2                  1

Are these good ideas?

I know about “the adjacency list model” and “the modified preorder tree traversal algorithm”, but are these good solutions for a “referral” system?

The queries that I need to perform on this tree are:

• frequently adding a new users
• when a user buys something, their referrers get a percentage commission
• every user should be able to find out how many people they’ve referred (and how many people were referred by people who they referred….) at each level

Answer 1

Closure Table

ancestor_id  descendant_id  distance
    1            1              0
    2            2              0
    3            3              0
    4            4              0
    5            5              0
    6            6              0
    2            3              1

To add user 10, referred by user 3. (I don’t think you need to lock the table between these two insertions):

insert into ancestor_table
select ancestor_id, 10, distance+1
from ancestor_table
where descendant_id=3;

insert into ancestor_table values (10,10,0);

To find all users referred by user 3.

select descendant_id from ancestor_table where ancestor_id=3;

To count those users by depth:

select distance, count(*) from ancestor_table where ancestor_id=3 group by distance;

To find the ancestors of user 10.

select ancestor_id, distance from ancestor_table where descendant_id=10;

The drawback to this method is amount of storage space this table will take.