title: Remove rows from data.table in R based on values of several columns
body: <p>I have a data.table in R which has several ids and a value. For each combination of ids, there are several rows. If one of these rows contains NA in the column 'value', I would like to remove all rows with this combination of ids. For example, in the table below, I would like to remove all rows for which <code>id1 == 2</code> and <code>id2 == 1</code>.</p>
  
  <p>If I had only one id I would do <code>dat[!(id1 %in% dat[is.na(value),id1])]</code>. In the example, this would remove all rows where i1 == 2. However, I did not manage to include several columns.</p>
  
  <pre><code>dat &lt;- data.table(id1 = c(1,1,2,2,2,2),
                    id2 = c(1,2,1,2,3,1),
                    value = c(5,3,NA,6,7,3))
  </code></pre>