title: Reference to a column in the aggregation context may not appear outside of an aggregate function
body: <p>I need to build a query that calculates the percentage rate of each value of a category( in this case strategy) out of the total runs, across different time buckets.
  I came up with the below query, however I get syntax error :</p>
  <p><strong>Semantic error: Reference to a column 'total' in the aggregation context may not appear outside of an aggregate function.</strong></p>
  <pre><code>requests
  | where timestamp between (startTime..endTime)
  | extend strategy = tostring(customDimensions.QueryStrategy)
  | summarize count() by strategy, bin(timestamp, 30m)
  | join (requests 
  | where timestamp between (startTime..endTime)
  | summarize total = count() by bin(timestamp, 30m)) on timestamp
  | project timestamp, strategy, count_, total = toint(total)
  | summarize 100 *count()/total by strategy, timestamp
  </code></pre>