Difference between CROSS APPLY and OUTER APPLY

          SQL Server 2005 introduced the APPLY operator, which is very much like a join clause and which allows joining between two table expressions i.e. joining a left/outer table expression with a right/inner table expression. The difference between join and APPLY operator becomes evident when you have a table-valued expression on the right side and you want this table-valued expression to be evaluated for each row from the left table expression.
       The APPLY operator comes in two variants, the CROSS APPLY and the OUTER APPLY. The CROSS APPLY operator returns only those rows from left table expression (in its final output) if it matches with right table expression. In other words, the right table expression returns rows for left table expression match only.  Whereas the OUTER APPLY operator returns all the rows from left table expression irrespective of its match with the right table expression.  For those rows for which there are no corresponding matches in right table expression, it contains NULL values in columns of right table expression. So you might now conclude, the CROSS APPLY is semantically equivalent to INNER JOIN (or to be more precise its like a CROSS JOIN with a correlated sub-query) with a implicit join condition of 1=1 whereas OUTER APPLY is semantically equivalent to LEFT OUTER JOIN.

              You might be wondering if the same can be achieved with regular JOIN clause then why and when to use APPLY operator? Though the same can be achieved with normal JOIN, the need of APPLY arises if you have table-valued expression on right part and also in some cases use of APPLY operator boost the performance of your query.

e.g  
   SELECT * FROM Department D
CROSS APPLY
   (
   SELECT * FROM Employee E
   WHERE E.DepartmentID = D.DepartmentID
   ) A 

GO      SELECT * FROM Department D
OUTER APPLY
   (
   SELECT * FROM Employee E
   WHERE E.DepartmentID = D.DepartmentID
   ) A
GO