title: evaluate expression in MySQL
body: <p>I have a table with column a,b, expr  that expr is an expression from other column.
  for example table contains </p>
  
  <pre><code>a   b   expr
  ------------
  2   5   a+b
  3   4   a*b+3
  </code></pre>
  
  <p>I like simply run a query and get flowing result:</p>
  
  <pre><code>a | b | expr 
  ------------ 
  2 | 5 | 7 
  3 | 4 | 15
  </code></pre>
  
  <p>I search doc for function, procedure etc, but i can't do it.
  please help me!</p>