title: ISA Relationship in DBMS
body: <p>I am reading Introduction to database managements system written by Ramakrishnan, and there i faced the following ER diagram:</p>
  
  <p><img src="https://i.stack.imgur.com/13y38.jpg" alt="enter image description here"></p>
  
  <p>There, it says, </p>
  
  <pre><code>"For example, we might identify a subset of employees as Senior Emps. We can 
  modify Figure 2.12 to reflect this change by adding a second ISA node as a 
  child of Employees and making Senior Emps a child of this node."
  </code></pre>
  
  <p>My question is, why do we need another ISA node? Why don't we just add this new entity to the current ISA node? Does that matter?</p>
  
  <p>Thanks</p>