title: "null" in mathematical calculations?
body: <p>I'm trying to make a MySQL query that returns rows where <code>(col_a+col_b-col_c+col_d) != col_e</code>, where all of the columns are <code>decimal</code> and default to <code>null</code>. There is one row that I know of that meets these requirements, but when I ran the query with the above logic as the <code>WHERE</code> clause, the row didn't show up. I noticed that <code>col_c</code> was <code>null</code>, instead of a numerical value, and after changing it to <code>0</code> the row showed up when I ran the query.</p>
  
  <p>Why did this happen? I have always assumed that <code>null</code> was interpreted as <code>0</code> in an instance such as the above?</p>