title: extract from a data frame in R
body: <p>This is my principal data : </p>
  
  <pre><code>1    Cl1  28    2750 2015-05-16 Marseille        S7
  2    Cl1  27    2570 2015-06-03 Marseille        S7
  3 Cl1000  24    1950 2015-07-05 Marseille       S17
  4 Cl1000  17    1400 2016-01-09 Marseille       S17
  5  Cl104  29    2680 2015-01-02  Grenoble        S3
  6  Cl110  29    2660 2016-02-02    Calais        S2
  </code></pre>
  
  <p>I want to extract the quantities Qte per month, of the year 2015 and Typologies S1 and S2.</p>
  
  <pre><code>df&lt;-sqldf("select Qte, typologie  from 
            where ='S1, S2'
            And data_achat.year = 2015'
            Group by date_achat.month  
             ")  
  </code></pre>
  
  <p>I get Null as a result.</p>
  
  <p>I want also to visualise and compare quantities per month, which plot should I use?</p>