title: combining join with "or" in data.table package
body: <pre><code>dt &lt;- data.table(X=rnorm(10),a=rep(0:1,length=10),b=rep(0:1,each=5))
  dt
               X a b
  1:  0.08848742 0 0
  2: -1.36578648 1 0
  3: -1.01563937 0 0
  4:  0.36562936 1 0
  5:  2.04250239 0 0
  6:  1.33698124 1 1
  7: -1.38358719 0 1
  8: -0.14395236 1 1
  9: -1.36277622 0 1
  10:  0.40818281 1 1    
  
  setkey(dt,a,b)
  dt[J(1,1),]
  </code></pre>
  
  <p>This is a way to get all lines where both a and b are 1. Is there a way to pick those lines where either a or b is 1 ? In other words: to get all lines in dt except for line 1,3 and 5?</p>