title: add column to table in kdb based of existing columns?
body: <p>I want to add a new column to a kdb table, it should add based of the existing column by populating with the non null value as below</p>
  
  <pre><code>q)t:([]a:`a`b`c`d`e`f`g`h;b:1 0n 3 4 0n 6 0n 8;c:0n 2 0n 0n 5 0n 7 0n)
  q)t
  a b c
  -----
  a 1  
  b   2
  c 3  
  d 4  
  e   5
  f 6  
  g   7
  h 8  
  </code></pre>
  
  <p>I want to add a column d that would take the value from c or d that isn't null
  to produce a table like this</p>
  
  <pre><code>a b c d
  -------
  a 1   1
  b   2 2
  c 3   3
  d 4   4
  e   5 5
  f 6   6
  g   7 7
  h 8   8
  </code></pre>
  
  <p>I tried concatenating but then it has the null in it:</p>
  
  <pre><code>q)update d:(b,'c)from t
  a b c  d  
  ----------
  a 1    1  
  b   2    2
  c 3    3  
  d 4    4  
  e   5    5
  f 6    6  
  g   7    7
  h 8    8  
  </code></pre>