Merge Join (SSIS)

The Merge Join transformation provides an output that is generated by joining two sorted datasets using a FULL, LEFT, or INNER join. For example, you can use a LEFT join to join a table that includes product information with a table that lists the country/region in which a product was manufactured. The result is a table that lists all products and their country/region of origin. For more information, see Using Joins.
You can configure the Merge Join transformation in the following ways:

• Specify the join is a FULL, LEFT, or INNER join.
• Specify the columns the join uses.
• Specify whether the transformation handles null values as equal to other nulls. 

The Merge Join Transformation requires sorted data for its inputs.

http://msdn.microsoft.com/en-us/library/ms141775.aspx