title: Count the number of occurrences of Null in a column in tableau
body: <p>I am relatively new to Tableau and I am wondering if there is a way to calculate null values in a column.
  I have a column called Email of type string and want to know how many people have not entered their email i.e. Null.</p>
  
  <p>I tried to create a calculated field with 
  count(ISNULL([Email]))</p>
  
  <p>But this gives me the total count and not the count of null.</p>
  
  <p>Thanks.</p>