title: merge multiple dataframes into one using pandas
body: <p>I have one data frame df:</p>
  
  <pre><code>   fruit      date    volume
  0  apple    20141001    2000
  1  apple    20141101    1800
  2  apple    20141201    2200
  3  orange   20141001    1900
  4  orange   20141101    2000
  5  orange   20141201    3000
  ….
  </code></pre>
  
  <p>and I have following two data frames </p>
  
  <p>apple:</p>
  
  <pre><code>   date       price
  0  20141001   2
  1  20141101   2.5
  2  20141201   3
  </code></pre>
  
  <p>orange:</p>
  
  <pre><code>   date       price
  0  20141001   1.5
  1  20141101   2
  2  20141201   2
  </code></pre>
  
  <p>how can I merge all these in to the following data frame:</p>
  
  <pre><code>   fruit      date    price    volume
  0  apple    20141001   2       2000
  1  apple    20141101   2.5     1800
  2  apple    20141201   3       2200
  3  orange   20141001   1.5     1900
  4  orange   20141101   2       2000
  5  orange   20141201   2       3000
  ….
  </code></pre>
  
  <p>This is just a example, in my real work, I have hundreds of 'fruit' with price data need to be merged into the first data frame.</p>
  
  <p>should I use merge or join? what is the difference between them? Thank you.</p>