title: min/max on differences of DataFrame columns in Julia
body: <p>When I try to retrieve the maximums of differences of columns in a DataFrame I get an error. What is wrong?</p>
  
  <pre><code>using DataFrames
  
  a = [2,4,10,4,8,8]
  b = [5,9,7,2,8,7]
  c = [2,9,7,6,8,1]
  
  df = DataFrame(A = a, B = b, C = c)
  df[2,:A] = NA
  df[3,:C] = NA
  
  ab=df[:A] - df[:B]
  bc=df[:B] - df[:C]
  ac=df[:A] - df[:C]
  
  df[:max] = max(ab, bc, ac)
  
  println(df)
  </code></pre>
  
  <p>=> LoadError: MethodError: no method matching isless(::DataArrays.DataArray{Int64,1}, ::Array{Any,1})</p>
  
  <p>Doing the maximum of either <code>df[:max] = max(ab, bc)</code> or <code>df[:max] = max(a, b, c)</code> works as expected.</p>
  
  <p>Can anybody clarify what's going on? Thank you!</p>