title: Combine and update table in sql
body: <p>table test:</p>
  
  <pre><code>+--------+--------+-------+------+
  | source | target | count | prob |
  +--------+--------+-------+------+
  | test1  | test2  |     2 |    1 |
  | cat    | dog    |     2 |    1 |
  +--------+--------+-------+------+
  </code></pre>
  
  <p>table test2:</p>
  
  <pre><code>+--------+--------+-------+------+
  | source | target | count | prob |
  +--------+--------+-------+------+
  | cat    | dog    |     1 |  0.5 |
  | dog    | cat    |     1 |  0.5 |
  +--------+--------+-------+------+
  </code></pre>
  
  <p>I tried to combine two table by code blew </p>
  
  <pre><code>select test.count+test2.count, test.prob+test.prob 
  from test, test2
  where (test.source = test2.source) and (test.target = test2.target);
  </code></pre>
  
  <p>if i want update test table like this</p>
  
  <pre><code>+--------+--------+-------+------+
  | source | target | count | prob |
  +--------+--------+-------+------+
  | test1  | test2  |     2 |    1 |
  | cat    | dog    |     3 |   1.5|
  | dog    | cat    |     1 |   0.5|
  +--------+--------+-------+------+
  </code></pre>
  
  <p>Is any suggestion?</p>