title: How to sum the average over a month
body: <p>I am trying to sum data relating to database table sizes and can't seem to do what I need to do. I capture database name, table name and table size every day. Example data would look like this:</p>
  
  <pre><code>Database Name  Table Name  Date        Size
  Database 1     Table 1     01/01/2016  0.01
  Database 1     Table 1     02/01/2016  0.02
  Database 1     Table 1     03/01/2016  0.02
  Database 1     Table 2     01/01/2016  0.41
  Database 1     Table 2     02/01/2016  0.42
  Database 1     Table 2     03/01/2016  0.43
  Database 2     Table 1     01/01/2016  0.01
  Database 2     Table 1     01/01/2016  0.01
  Database 2     Table 1     01/01/2016  0.01
  </code></pre>
  
  <p>I want to produce a simple line chart showing database size over time. If I add database name in row, Date in column and Size in value, I get a line chart showing sum of Size per database. This gives the correct database size when viewed at daily level, as the correct database size value at that granularity is simply the sum of size of all tables within each database. With the above data, for example, Database 1 on 01/01/2016 should be 0.42.</p>
  
  <p>However, when I look at month level, incorrect amounts are shown. Tableau is summing all values for each database in each month. So in the above example for month January 2016 and Database 1 I get a value of 1.31. I need to get Tableau to get the average value for each table in each month, then sum all of those average table sizes per database to get the average total database size for that month.</p>
  
  <p>I'm, sure it's possible but I can't find a way to do it.</p>