title: Sequential left Joins behave as inner Joins
body: <p>I have the following data in 3 tables:</p>
  
  <pre><code>  materials          customers             sales
   -----------        -----------        --------------   
    mtrl comp         cust  comp        mtrl  cust  expqty
      1   2            22    2           1     22     1
      2   2            23    2           2     22     2
      3   2            24    2           3     23     3
      4   2            25    2           4     24     4
  </code></pre>
  
  <p>and I want to produce the following result:</p>
  
  <pre><code>mtrl  cust  expqty  
   1     22     1  
   2     22     2  
   3     22    null  
   4     22    null
  </code></pre>
  
  <p>In SQL Server I am using the following query:</p>
  
  <pre><code>select a.mtrl,b.cust,c.expqty from materials a left join customers b on a.comp=b.comp 
  left join sales c on c.mtrl=a.mtrl and c.cust=b.cust where b.cust=22
  </code></pre>
  
  <p>and although I am using a left join, I loose the last 2 lines and all I get is:</p>
  
  <pre><code>   mtrl  cust  expqty  
       1     22     1  
       2     22     2  
  </code></pre>
  
  <p>Can you advice me on what I am doing wrong or how would I achieve the desired result? </p>