title: Using DATE_ADD with a Column Name as the Interval Value
body: <p>I have a table which contains products, a start date and an interval value :</p>
  
  <pre><code>product_name                    start_date              expiry_period
  Domain Registration (1 Year)    2013-12-08 00:00:00     1 Year
  Domain Registration (1 Year)    2013-12-01 00:00:00     1 Year
  Website Hosting (Bronze)        2013-12-19 00:00:00     1 Year
  Website Hosting (Silver)        2013-12-20 00:00:00     1 Year
  Website Hosting (Silver)        2013-12-21 00:00:00     1 Year
  Domain Registration (2 years)   2014-01-04 00:00:00     2 Year
  Domain Registration (1 Year)    2014-01-04 00:00:00     1 Year
  Website Hosting (Silver)        2014-01-06 00:00:00     1 Year
  Domain Registration (2 years)   2014-01-06 00:00:00     2 Year
  Domain Registration (1 Year)    2014-01-07 00:00:00     1 Year
  Domain Registration (1 Year)    2014-01-10 00:00:00     1 Year
  Website Hosting (Bronze)        2014-01-12 00:00:00     1 Year
  </code></pre>
  
  <p>I'm trying to add a calculated value in my select statement to add the interval to the start_date so that my dataset returns with the start and end date programmatically.</p>
  
  <p>Here's what I have at the moment:</p>
  
  <pre><code>select 
      product_name, 
      start_date,
      expiry_period
      DATE_ADD(start_date, INTERVAL expiry_period) as end_date
  from
      tbl_products
  </code></pre>
  
  <p>However, it's returning an error against the DATE_ADD line (incorrect SQL Syntax).</p>
  
  <p>All of the SO articles that I've read seem to indicate that the expression and the type need to be separate (i.e. <code>DATE_ADD(start_date, INTERVAL expiry_value expiry_type)</code>) - Surely this isn't the case and I can just pass a period in one single field?</p>
  
  <p>If not and since I can't change the data schema, what would be the recommended way of doing it? I contemplated using <code>SUBSTRING_INDEX</code> to split the column by  but this doesn't appear to work either.</p>