title: SQL query to add values of two columns containing null values?
body: <p>Given table:</p>
  
  <pre>
      ID   ONE   TWO
      X1   15    15
      X2   10    -
      X3   -     20
  </pre>
  
  <p>This query:</p>
  
  <pre><code>SELECT (ONE + TWO) FROM (TABLE)
  </code></pre>
  
  <p>Just returns the sum of <code>X1</code>'s values but not the others since at least one column has a null value. How can I still add them even if there is a null? i.e. consider the null as a 0 maybe?</p>