title: Adding column based on ID in another data
body: <p>data1 is data from 1990 and it looks like</p>
  
  <pre><code>Panelkey   Region    income
   1           9           30
   2           1           20
   4           2           40
  </code></pre>
  
  <p>data2 is data from 2000 and it looks like</p>
  
  <pre><code>Panelkey   Region    income
    3          2           40
    2          1           30
    1          1           20
  </code></pre>
  
  <p>I want to add a column of where each person lived in 1990. </p>
  
  <pre><code>Panelkey    Region    income   Region1990
    3          2          40        .
    2          1          30        1
    1          1          20        9
  </code></pre>
  
  <p>How can I do this on Stata?</p>