title: Strange warning message: In FUN(X[[i]], ...) : NAs introduced by coercion
body: <p>I found a strange behaviour of sqldf - it issues "NAs introduced by coercion" warning when a variable exists with the same name as a column and different data type. The following code:</p>
  
  <pre><code>x &lt;- structure(list(euring = c(12380, 12430), species = c("Locustella luscinioides", 
  "Acrocephalus schoenobaenus")), .Names = c("euring", "species"
  ), row.names = 1:2, class = "data.frame")
  
  species &lt;- structure(list(EURING = c(0, 980), Species = c(NA_integer_, NA_integer_
  )), .Names = c("EURING", "Species"), row.names = 1:2, class = "data.frame")
  
  require(sqldf)
  
  result &lt;- sqldf("
  select species as Species
  from x
  ")
  </code></pre>
  
  <p>produces:</p>
  
  <blockquote>
    <p>Warning message: In FUN(X[[i]], ...) : NAs introduced by coercion</p>
  </blockquote>
  
  <p>and <code>result</code> then contains only NAs. </p>
  
  <p>If variable <code>species</code> didn't exist, everything is OK. But <code>sqldf</code> shoudln't even touch the variable <code>species</code>, right??</p>
  
  <p><strong>EDIT:</strong> I suspect it is a bug. I reported it: <a href="https://github.com/ggrothendieck/sqldf/issues/23" rel="nofollow noreferrer">https://github.com/ggrothendieck/sqldf/issues/23</a></p>