title: Defining a table: Allow nulls or stick with "n/a"
body: <p>I have a simple design question for defining a table. I have to read in strings which are numeric values but also can be "n/a". Now I have two options. I could define a column as varchar and allow "n/a" or I define such column as numeric ( float ) and allow nulls.</p>
  
  <p>Since I would like to do aggregation I tend to store the values as floats instead of casting them all the time. What's your opinion?</p>