title: SQL Query to match unlinked data
body: <p>Say I have three tables:</p>
  
  <pre><code>TABLE A
  
  idA     variable
  1       Number of hats
  2       Number of scarves
  3       Number of mittens
  
  
  TABLE B
  
  idB     name
  1       Andy
  2       Betty
  3       Cedric
  4       Daphne
  
  
  TABLE C
  
  idA     idB     value
  1       1       15
  1       2       2
  1       3       89
  2       1       10
  2       3       3
  2       4       1504
  3       2       12
  3       3       4
  3       4       1
  </code></pre>
  
  <p>Looking at the table, it's relatively simple to work out - we know how many hats (2) and mittens (12) that she owns, but not how many scarves. Likewise, for Daphne we know how many scarves (1504) and mittens (1) she owns, but not the amount of hats.</p>
  
  <p>However, I'd like a list of fields that there ISN'T information for - I would have a returned result looking something like this (if I asked for Andy)</p>
  
  <pre><code>idA     variable
  3       Number of mittens
  </code></pre>
  
  <p>Any idea how I do that? :)</p>