relationship between database tables not sharing any foreign key(s) in ERD
<p>Consider an example of following database tables:</p>

<blockquote>
  <p>user ( id [PK], user_name, ... )</p>
  
  <p>comment ( id [PK], u_id [FK], ... )</p>
  
  <p>reportedCommentInfo ( id [PK], c_id [FK], reporting_u_id [FK], date, ... )</p>
  
  <p>admin ( id [PK], user_name, ... )</p>
</blockquote>

<p>According to this database schema any user can post a comment and can report other user comments based on containing abusive or spamming material. Admin can view a list of all reported comments by users and can remove comments not complying to set standards.</p>

<p>In this scenario admin doesn't actually have access to any of the reported comments i.e. their id(s) are not present inside the admin table, yet admin can access and manage reported comments. So while creating ERD will their be a relation between reportedCommentInfo and admin table?</p>

<p>i.e. basically my question is, while creating ERD/data model can we create a relationship between two tables which do not have any primary key / foreign key relationship?</p>

<p>or two tables can only be related if they have primary key / foreign key relationship among them.</p>

<p><a href="https://i.stack.imgur.com/TUJYI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TUJYI.png" alt="ERD for above example"></a></p>

<p>p.s: any suggestions for improving anything in either database schema or ERD structure is more than welcome.</p>