title: data.table - group by all except one column
body: <p>Can I group by all columns except one using <code>data.table</code>? I have a lot of columns, so I'd rather avoid writing out all the <code>colnames</code>.</p>
  
  <p>The reason being I'd like to collapse duplicates in a table, where I know one column has no relevance.</p>
  
  <pre><code>library(data.table)
  
  DT &lt;- structure(list(N = c(1, 2, 2), val = c(50, 60, 60), collapse = c("A", 
  "B", "C")), .Names = c("N", "val", "collapse"), row.names = c(NA, 
  -3L), class = c("data.table", "data.frame"))
  
  &gt; DT
     N val collapse
  1: 1  50        A
  2: 2  60        B
  3: 2  60        C
  </code></pre>
  
  <p>That is, given <code>DT</code>, is there something like like <code>DT[, print(.SD), by = !collapse]</code> which gives:</p>
  
  <pre><code>&gt; DT[, print(.SD), .(N, val)]
     collapse
  1:        A
     collapse
  1:        B
  2:        C
  </code></pre>
  
  <p>without actually having to specify <code>.(N, val)</code>? I realise I can do this by copy and pasting the column names, but I thought there might be some elegant way to do this too.</p>