title: Mysql 1241 operand should contain 1 column s
body: <p>I am using scrapy to crawl information and store into mysql.</p>
  
  <p>Some relevant code are:</p>
  
  <pre><code>def _conditional_insert(self, tx, item):
      if item.get('app_name'):           
          tx.execute("""
              insert into af_appinfo (app_name, app_size, app_data_install, app_icon_url, app_download_url, app_desc) 
              values (%s, %s, %s, %s, %s, %s)""",
              (item['app_name'], item['app_size'], item['app_data_install'], item['icon_url'],item['app_url'], item['app_desc']))
  </code></pre>
  
  <p>The definition of item is:</p>
  
  <pre><code>from scrapy.item import Item, Field
  
  class WdjcrawlItem(Item):
    icon_url = Field()
    app_url = Field()
    app_name = Field()
    app_desc = Field()
    app_size = Field()
    app_data_install = Field()
  </code></pre>
  
  <p>The crawler works well. BUT  an exception is raised when using mysql. The exception information is :</p>
  
  <pre><code>_mysql_exceptions.OperationalError: (1241, 'Operand should contain 1 columns)
  </code></pre>
  
  <p>Any mistake about this query?</p>