title: Most efficient way to store "likes" like Facebook and Stackoverflow
body: <p>What would be the best way to store "likes" or "upvotes" in a mysql database?</p>
  
  <p>Here was what I was thinking:</p>
  
  <pre><code>likes (table)
      id (auto increment)
      post_id
      user_id
  </code></pre>
  
  <p>but would the <code>likes</code> table have millions or even billions of rows since each person that would like a post would insert into 1 row and each person can like more than one time and there's more than one user.</p>
  
  <p>how does facebook or stackoverflow manage to do this in a database? any links or guides is appreciated. thanks.</p>