title: Comparing data.table columns with NAs in numerical columns
body: <p>I just came across an odd observation today that <code>NA</code>s in a column inside a data.table don't return <code>TRUE</code> or <code>FALSE</code>, but just <code>NA</code>. This question <a href="https://stackoverflow.com/questions/57598576/comparing-column-values-with-na">Comparing Column Values With NA</a> has an answer, but it only refers to character type columns, where changing <code>NA</code> to <code>''</code> won't affect the data.
  Is there a way to compare the data and get back a <code>TRUE</code> or <code>FALSE</code> without changing the data type of the column?</p>
  <pre><code>library(data.table)
  dt &lt;- data.table(a = c(1:10)
         , b = c(1:4,NA,6:10))
  
  dt[a != b,]
  </code></pre>
  <p>In this case I would expect to get the 5th row, because 5 is not equal to NA in my opinion. If anyone could provide some background on why that happens, would also be great.</p>