title: MySQL Counting number of duplicate data in a table
body: <p>How do I get the number of dumplicate data in a table.</p>
  
  <p>Example: I have these datas in a table.</p>
  
  <p>ID: 1 2 3 4 5 <br />
  Score: 100 300 400 100 300</p>
  
  <p>I want the result to be 2 because 100 has a duplicate andalso 300, so that gives the result to be 2.</p>
  
  <p>I'm thinking of a group by and count aggregate function. But it won't work because it will include 400 even if it doesn't have duplicate data.</p>
  
  <p>How would I do t that?</p>
  
  <p>Your help would be greatly appreciated. Thanks!</p>