Change default key from `id` to another column

[rasadeghnasab]

I have a table which has these columns:

id, user_id, parent_id, sport_id

I want to create children using user_id column instead of id. for example:

id      user_id      parent_id       sport_id
1    20       null      2
2    25       20        2   # user#20 is user#25 parent.
3    28       25        2   # user#25 is user#28 parent.
4    29       20        2   # user#20 is user#29 parent.

please tell me what should I do?