title: NA values using sqldf
body: <p>If I try to get an average of <code>c(NA, NA, 3, 4, 5, 6, 7, 8, 9, 10)</code> using <code>AVG</code> from SQL, I get a value of 5.2, instead of the expected 6.5.</p>
  
  <pre><code># prepare data and write to file
  write.table(data.frame(col1 = c(NA, NA, 3:10)),
          "my.na.txt", row.names = FALSE)
  
  mean(c(NA, NA, 3:10), na.rm = TRUE) # 6.5
  
  my.na &lt;- read.csv.sql("my.na.txt", sep = " ",
          sql = "SELECT AVG(col1) FROM file") # 5.2
  
  # this is identical to
  sum(3:10)/10
  
  unlink("my.na.txt") # remove file
  </code></pre>
  
  <p>Which leads me to believe that sql(df) treats NA values as zero. Is it possible to ignore (exclude) NA values in an SQL call as it can be done using <code>na.rm</code> argument (in R)?</p>