title: Calculate multiple field including null field
body: <p>The calculation query only works when all the field were filled, it can correctly calculate 0 value but when there is <em>NULL</em> value, it doesnt returning any result</p>
  
  <p>query :</p>
  
  <pre><code>SELECT
  no,
  price_1,
  price_2,
  price_3,
  coalesce(round(price_1+price_2+price_3,2),0) AS total
  from price_table
  group by no
  </code></pre>
  
  <p>i use round to simplify the calculated result up to 2 decimal only </p>
  
  <p>table :</p>
  
  <pre><code>++++++++++++++++++++++++++++++++++++             ++++++++++++++++++++++++++++++++++++++++++++
  | no | price_1 | price_2 | price_3 |             | no | price_1 | price_2 | price_3 | total |
  ++++++++++++++++++++++++++++++++++++             ++++++++++++++++++++++++++++++++++++++++++++
  | 1  |   0.10  |  NULL   |  NULL   |  result =&gt;  | 1  |   0.10  |  NULL   |  NULL   |       |
  | 2  |   0.20  |    0    |    0    |             | 2  |   0.20  |    0    |    0    |  0.20 |
  | 3  |   0.30  |   0.20  |   0.50  |             | 3  |   0.30  |   0.20  |   0.50  |  1.00 |
  ++++++++++++++++++++++++++++++++++++             ++++++++++++++++++++++++++++++++++++++++++++
  </code></pre>
  
  <p>result i looking for :</p>
  
  <pre><code>++++++++++++++++++++++++++++++++++++             ++++++++++++++++++++++++++++++++++++++++++++
  | no | price_1 | price_2 | price_3 |             | no | price_1 | price_2 | price_3 | total |
  ++++++++++++++++++++++++++++++++++++             ++++++++++++++++++++++++++++++++++++++++++++
  | 1  |   0.10  |  NULL   |  NULL   |  result =&gt;  | 1  |   0.10  |  NULL   |  NULL   |  0.10 |
  | 2  |   0.20  |    0    |    0    |             | 2  |   0.20  |    0    |    0    |  0.20 |
  | 3  |   0.30  |   0.20  |   0.50  |             | 3  |   0.30  |   0.20  |   0.50  |  1.00 |
  ++++++++++++++++++++++++++++++++++++             ++++++++++++++++++++++++++++++++++++++++++++
  </code></pre>
  
  <p>my question is on no.1 row , the total field is not showing any result, because the value of price_2 and/or price_3 is NULL, how to make the query to calculate all the price_  field including NULL value correctly?</p>