title: Joining three table in mysql
body: <p>I have three tables like below.</p>
  
  <pre><code>Courses:
  Id Name
  1  A
  2  B
  3  C
  4  D
  
  Prices:
  Id CId Price
  1   1   200
  2   2   150
  3   3   500
  4   4   300
  
  Payment:
  Id  CId
  1    1
  2    4
  </code></pre>
  
  <p>If a record of payment for Course record exists, then Price should be -1.  I want to get result like below:</p>
  
  <pre><code>Id Name Price
  1   A    -1
  2   B    150
  3   C    500
  4   D     -1
  </code></pre>
  
  <p>How should I query mysql?</p>