title: SQL query to select posts belonging to multiple categories
body: <p>I am writing a web application similar to a blogging software. 
  There are three tables as below</p>
  
  <pre><code>Posts Table: Post_id,Post_Text
  Post_Tags Table: Post_id,Tag_id
  Tags Table:Tag_id,Tag_name
  </code></pre>
  
  <p>I have a difficulty in conceptualizing a SQL query that will return posts that have 'all of the' tags in a given set.</p>