title: How to drop rows from a dataframe as per null values in a specific column?
body: <p>How to drop rows from a dataframe as per null values in a specific column?</p>
  
  <p>Say I have a dataframe that has three columns a,b,c and all can have null values, but I only want to droprows where column b has null/NaN. How can I do that in pandas dataframe?</p>