title: How to select distinct value from two columns into one column
body: <p>There are two location related columns in my table like below</p>
  <pre><code>| Service_No | A_LOC | Z_LOC |
  |------------|-------|-------|
  | 001        | A     | B     |
  | 002        | A     | C     |
  | 003        | Null  | C     |
  | 004        | F     | B     |
  </code></pre>
  <p>How do I select the distinct values of columns <code>A_LOC</code> and <code>Z_LOC</code> combined into a single list? The result of this query would be:</p>
  <pre><code>A, B, C, F
  </code></pre>