Query to one parent record from multiple child record

Hi, Thanks in advance,I need help in creating SQL query to generate report to get one parent account for multiple child record( db - postgres db ).

example below with below rules

  1. For each "Id" consider lowest "sub_id" as parent_id and other as child_id
  2. The parent_id shoul be same for all child_id
    I.e. no child_id should have multiple parent_id
  3. The parent_id and child_id can't be same

Id sub_id
1 a11
1 a12
2 a13
2 a16
2 a17
2 a14
3 a31
4 a32
4 a33
4 a35

Final result
Parent_id child_id
a11 a12
a13 a14
a13 a16
a13 a17
a32 a33
a32 a35