title: if condition in where clause of sql query
body: <p>Below is my where clause of the query when i want data for a specific user</p>
  
  <pre><code>where Completion_Date&gt;= '11/01/2011' 
  and Completion_Date&lt;= '12/11/2012' 
  and System_user_id = 1234
  </code></pre>
  
  <p>and below is the where clause when i want to pull data for all the user:</p>
  
  <pre><code>where Completion_Date&gt;= '11/01/2011' 
  and Completion_Date&lt;= '12/11/2012'
  </code></pre>
  
  <p>Since i dont want 2 seperate queries, is there a way to add a condition in where clause so that i can use a single query and depending on the input (i.e System_user_id) it will decide on whether to add extra condtion in the query.
  I will be sending -1 when i want data for all users &amp; for a specific user its system_user_id will be sent.</p>