title: SELECT Current and Previous row WHERE condition
body: <pre><code>id value
  ---------
  1 a
  2 b
  3 c
  4 a
  5 t
  6 y
  7 a
  </code></pre>
  
  <p>I want to select all rows where the value is 'a' and the row before it</p>
  
  <pre><code>id value
  ---------
  1 a
  3 c
  4 a
  6 y
  7 a
  </code></pre>
  
  <p>I looked into </p>
  
  <p>but I want to get all such rows in one query.</p>
  
  <p>Please help me start</p>
  
  <p>Thank you</p>