title: How to get the number of entries in a measurement
body: <p>I am a newbie to influxdb. I just started to read the influx documentation.</p>
  
  <p>I cant seem to get the equivalent of <em>'select count(*) from table'</em> to work in influx db. </p>
  
  <p>I have a measurement called cart:</p>
  
  <pre><code>time                status  cartid          
  1456116106077429261 0       A
  1456116106090573178 0       B
  1456116106095765618 0       C
  1456116106101532429 0       D
  </code></pre>
  
  <p>but when I try to do</p>
  
  <pre><code>select count(cartid) from cart
  </code></pre>
  
  <p>I get the error </p>
  
  <pre><code>ERR: statement must have at least one field in select clause
  </code></pre>