title: Oracle: What does `(+)` do in a WHERE clause?
body: <p>Found the following in an Oracle-based application that we're migrating <em>(generalized)</em>:</p>
  
  <pre><code>SELECT
      Table1.Category1,
      Table1.Category2,
      count(*) as Total,
      count(Tab2.Stat) AS Stat
  FROM Table1, Table2
  WHERE (Table1.PrimaryKey = Table2.ForeignKey(+))
  GROUP BY Table1.Category1, Table1.Category2
  </code></pre>
  
  <p>What does <code>(+)</code> do in a WHERE clause? I've never seen it used like that before.</p>