title: Selecting Subset of Pandas DataFrame
body: <p>I have two different pandas DataFrames and I want to extract data from one DataFrame whenever the other DataFrame has a specific value at the same time.To be concrete, I have one object called "GDP" which looks as follows: </p>
  
  <pre><code>               GDP
  DATE               
  1947-01-01    243.1
  1947-04-01    246.3
  1947-07-01    250.1
  </code></pre>
  
  <p>I additionally have a DataFrame called "recession" which contains data like the following:</p>
  
  <pre><code>            USRECQ
  DATE         
  1949-07-01       1
  1949-10-01       1
  1950-01-01       0
  </code></pre>
  
  <p>I want to create two new time series. One should contain GDP data whenever USRECQ has a value of 0 at the same DATE. The other one should contain GDP data whenever USRECQ has a value of 1 at the same DATE. How can I do that? </p>