title: Understanding OUTER JOIN for DateTimes, Tableau
body: <p>I am trying to understand the uses and limitation of outer joins in tableau (tableau online in this case). I have found the beavaiour of tableau to be not what I have expected.
  I have provided as detailed a description to my problems below, to avoid any ambiguotity and since I don't know where to start anymore. I hope I have not gone overboard(edits welcome).</p>
  
  <h1>Specifics of my use case</h1>
  
  <p>I am creating a join between two .csv files that have logged natural data at specific time intervals. One set has hourly time intervals, the other at intervals of minutes (which is variable due to various factors).</p>
  
  <p><strong>'Rain' data set(1):</strong><br>
  Date and Time   <code></code> <code></code> |  Rain<br>
  01/01/2018 00:00    <code></code>| 0<br>
  01/01/2018 01:00    <code></code>| 0.4<br>
  01/01/2018 02:00    <code></code>| 1.4<br>
  01/01/2018 03:00    <code></code>| 0.4  </p>
  
  <p><strong>'Fill' data set (2):</strong><br>
  Date and Time  <code></code> <code></code>|  Fill<br>
  24/04/2018 06:04  <code></code>| 78<br>
  24/04/2018 12:44  <code></code>| 104<br>
  24/04/2018 18:51  <code></code>| 96<br>
  25/04/2018 00:20  <code></code>| 84  </p>
  
  <p>Unsurprisingly, I have many nulls in the data (which is not a problem to me) as:  </p>
  
  <ul>
  <li>'Rain' has a longer time series</li>
  <li>In either data set, the majority of date times do not have an exact equivalent in the other</li>
  </ul>
  
  <p><a href="https://i.stack.imgur.com/gc5rD.png" rel="nofollow noreferrer">screenshot of data join here</a></p>
  
  <h1>What I am trying to achieve</h1>
  
  <p>I am trying to graph the two data sets in such a way that that I can compare the full data sets against each other, in all of the following ways:  </p>
  
  <ul>
  <li>Monthly or Yearly aggregation (average)</li>
  <li>Hourly aggregation (average)</li>
  <li>Exact times</li>
  </ul>
  
  <h1>Problems (and my limited assumptions)</h1>
  
  <ol>
  <li>Once graphed in tableau some values had <code>'null'</code> DateTime values<code>*</code>.</li>
  <li>Once graphed in tableau it appears as if many points are simply missing<code>**</code>
  <a href="https://i.stack.imgur.com/qa17c.png" rel="nofollow noreferrer">Graphing using 'Fill' time series</a><br>
  <a href="https://i.stack.imgur.com/q0ZGb.png" rel="nofollow noreferrer">Graphing using 'Rain' time series</a></li>
  <li>I had assumed (giving the full outer join of <code>'Date and Time(s)'</code>) tableau would join the data sets in chronological order with a common date time series </li>
  </ol>
  
  <p><code>*</code> I had assumed it impossible for the join conditions to have <code>'null'</code> values without throwing an error. Also, the data is clean and uniform<br>
  <code>**</code> And this is when aggregating monthly, which I assumed would not be affected by any (if any) hourly/minute mismatches</p>
  
  <h1>So, finally the question #</h1>
  
  <p>In my reading of the online help documentation I am struggling to find a functionality that is native to tableau that can help me achieve these specific goals. I am reaching the worrying conclusion that tableau was not built for this type of 'visual analytics'.<br>
  Is there a functionality native to tableau that will allow me to combine the data in the way I described above?</p>
  
  <h1>Approaches I have considered</h1>
  
  <p>Since I have two <code>.csv</code> files I could combine both set so that I have the full, granular <code>'Date and Time'</code> fields in one tall list.<br>
  However, I would like to find a method that is natural to tableau (online) because in future, at least some of the data wil come from a database (postgres) connection <strong>but</strong> others will likely have to remain as upload as a <code>.csv</code>  or <code>excel</code> files.</p>
  
  <h1>Again I ask</h1>
  
  <p>What am I overlooking in regards how (and why) to use tableau?
  I am not looking for a complete solution, but what tools could I use to achieve this?</p>
  
  <p>Many thanks for any help</p>