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?