title: update table cells to only have unique values
body: <p>I have a database table with a grade_range column, the rows of this column contain duplicate values all separated by a comma (like a list).</p>
  
  <pre><code>grade_range
  "Pre-K, Pre-K, Pre-K"
  "Pre-K, K-9, K-9"
  "Pre-K, K-7, 9-12"
  "Pre-K, K-7, K-7"
  "Pre-K, K-6"
  "Pre-K, K-5"
  </code></pre>
  
  <p>I want to update these records so that I am only left with the unique values in the rows of this column, like this</p>
  
  <pre><code>grade_range
  "Pre-K"
  "Pre-K, K-9"
  "Pre-K, K-7, 9-12"
  "Pre-K, K-7"
  "Pre-K, K-6"
  "Pre-K, K-5"
  </code></pre>
  
  <p>Distinct column values can be easily selected in SQL using - </p>
  
  <pre><code>SELECT DISTINCT grade_range FROM dev.school
  </code></pre>
  
  <p>but, I am not sure how to select/update distinct values inside a row-cell. One way I thought of doing was to manipulate this data in Excel and then updating records using an <code>UPDATE</code> statement based on the same <code>sid</code>s</p>
  
  <p>Any suggestion on accomplishing this in SQL?</p>