title: are "Unidirectional" and "Bidirectional" terms belong to OOP world or database world? is everything in Database bi-directional?
body: <p>I'm trying to learn about JPA and Hibernate and I was trying to learn about some database terms.</p>
  <p>I've question about &quot;Unidirectional&quot; and &quot;Bidirectional&quot; terms. when I search on internet, I see some posts and some articles that use these two terms for 'entity' and 'object'. so I assumed that these two words belong to OOP world.</p>
  <p>ok. but in <a href="https://www.youtube.com/watch?v=nf1vwRpHNW0" rel="nofollow noreferrer">this video</a>, the teacher (at 1:49) said:</p>
  <blockquote>
  <p>everything in Database is is bi-directional. because based on Foreign
  key, you can reach the value from either side.</p>
  </blockquote>
  <p>so it means that we can use Unidirectional and Bidirectional in database too. right?
  but I think the quote from the teacher is not right. because if we have <code>tableA</code> which has a foreign key to <code>tableB</code> and <code>tableB</code> does not have any foreign key to tableA, then it is Unidirectional because we cannot navigate and go from a <code>tableB</code>'s record to a <code>tableA</code>'s record.</p>
  <p>Could you please help me with this issue? thank you</p>
  <h2>Edit:</h2>
  <p><strong>PS</strong>: can we say if we use <code>JOIN</code>, we can say that the teacher is right? If for example the Person class would have a table which holds a foreign key that references the address table, then only the person table will know about the address. The address table will know about the person table only if it joins the table</p>