title: How to merge two data frames with different sizes
body: <p>I am working on two data frames with different sizes.</p>
  
  <pre><code>matA : col1 col2 col3
  row1   aa    abc   123
  row2   cc    dfg   455
  row3   ee    efg   345
  
  matB : col1 col4 col5
  row1   aa    a1   b1
  row2   cc    a2   b2
  row3   dd    a3   b3
  row4   ee    a4   b4
  
  dim(matA) : 2000 * 3
  dim(matB) : 4000 * 3
  
  
  matC : col1 col2 col3  col4 col5
  row1 : aa   abc  123   a1    b1
  row2 : cc   dfg  455   a2    b2
  row3 : dd   efg  345   a3    b3
  row4 : ee   NA   NA    a4    b4
  </code></pre>
  
  <p>I'd merge two into a combined mat(matC) but size is equal as bigger mat(here matB), which some of not matched rows to mat1 should be just empty or NA in matC.</p>
  
  <p>Wonder how to merge above two data frames?</p>