title: Select all records from a 3 column table where there is duplication in 2 specific columns
body: <p>Let's say I have the following table in SQL Server (tbl1):</p>
  
  <pre><code>Num     Zip     Rating
  1       75235   100
  2       77234   50
  3       77234   100
  4       77234   100
  5       77234   100
  6       75235   75
  7       76076   25
  8       76076   25 
  9       76076   50
  10      75234   25
  11      75234   50
  12      75234   50
  </code></pre>
  
  <p>I want to select all of the rows where the Zip and Rating columns combined have duplicates. In this case, that would be rows 3, 4, 5, 7, 8, 11, and 12. How would I do that?</p>