title: SQL JOIN avoid duplicate in column value
body: <p>I'm trying to create a view with join on a specific column. When duplicates on the joining column exist in 1 of the tables, the join duplicates the values on the opposite table where there is no duplicates. I need to show the values but only match it once, if a second match is found, the non-duplicated table should show null. Is it possible?</p>
  
  <pre><code>CREATE VIEW view1 AS
  SELECT A.id as id_a, B.id as id_b   
  FROM table1 A
  FULL OUTER JOIN table2 B ON  A.columnX = B.columnX --- the duplicate is in this column
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/wqkUp.png" alt="Expected result"></p>