title: Merging tables of different length by common columns R
body: <p>I've got two tables of different length and I need to merge them by two common columns (season and client), and fill cells with NAs when there isn't common elements. Below I'm showing a small fraction of the  two original tables and the final table that I need. I've tried many things with no success.</p>
  
  <pre><code>season  client.ID   qtty
  1998    13  30
  1999    13  30
  2000    13  29
  1998    28  18
  1999    28  18
  2000    28  18
  1998    35  21
  1999    35  21
  2000    35  21
  
  season  client.ID   vessel.ID   overLength
  1998    28  29  17.1
  1998    28  1809    4.26
  1998    28  2215    9.45
  1998    28  4173    5.8
  1998    28  8151    4.5
  1999    28  29  17.1
  1999    28  1809    4.26
  1999    28  2215    9.45
  1999    28  4173    5.8
  1999    28  8151    4.5
  2000    28  29  17.1
  2000    28  1809    4.26
  2000    28  2215    9.45
  2000    28  4173    5.8
  2000    28  8151    4.5
  1998    35  36  9.91
  1999    35  36  9.91
  2000    35  36  9.91
  1998    35  40  9.91
  1999    35  40  9.91
  2000    35  40  9.91
  
  
  season  client.ID   vessel.ID   overLength  qtty
  1998    13  NA  NA  30
  1999    13  NA  NA  30
  2000    13  NA  NA  29
  1998    28  29  17.1    18
  1998    28  1809    4.26    18
  1998    28  2215    9.45    18
  1998    28  4173    5.8 18
  1998    28  8151    4.5 18
  1999    28  29  17.1    18
  1999    28  1809    4.26    18
  1999    28  2215    9.45    18
  1999    28  4173    5.8 18
  1999    28  8151    4.5 18
  2000    28  29  17.1    18
  2000    28  1809    4.26    18
  2000    28  2215    9.45    18
  2000    28  4173    5.8 18
  2000    28  8151    4.5 18
  1998    35  36  9.91    21
  1999    35  36  9.91    21
  2000    35  36  9.91    21
  1998    35  40  9.91    21
  1999    35  40  9.91    21
  2000    35  40  9.91    21
  </code></pre>