title: Filter with multivalue columns in Redshift/SQL
body: <p>I have a table with news articles. These news articles have many columns that describe title, picture and so on. Some of the columns can have multiple values, e.g. category can be set to both "sports" and "hockey".</p>
  
  <p>Lets say I have this table:</p>
  
  <pre><code>articlekey | category
  ---------------------
  article1   | sports, hockey  
  </code></pre>
  
  <p>The actual table contains a lot of articles and all the articles are present only once. What I try to achieve is to filter on this table, on both values of category. To be able to do that, I have split them into to rows and generated a filter-table like this:</p>
  
  <pre><code>articlekey | category
  ---------------------
  article1   | sports
  article1   | hockey
  </code></pre>
  
  <p>(btw. we use Tableau as visualization/BI-tool, and that is where I filter)</p>
  
  <p>When I left join these to tables and filter (include) only on "hockey", I will get the correct result, since article1 only have one row with category set to "hockey".</p>
  
  <pre><code>articlekey | category         | category-filter
  -----------------------------------------------
  article1   | sports, hockey   | sports          &lt;-- this will be excluded
  article1   | sports, hockey   | hockey          &lt;-- this is included
  </code></pre>
  
  <p>But if I try to exclude "hockey", the article will show up with category set to "sports", since it persists in the result with category "sports". The result I want it to exclude the article entirely.</p>
  
  <pre><code>articlekey | category         | category-filter
  -----------------------------------------------
  article1   | sports, hockey   | sports          &lt;-- this is included, but should also be gone
  article1   | sports, hockey   | hockey          &lt;-- this will be excluded
  </code></pre>
  
  <p>If possible, how should I work with data like this, when I have multiple values per column and need to filter (include and exclude) so that I only have one row per article left.</p>