Understanding Views Relationships
Source: http://drupal.org/node/1578586 Relationships are used to allow Views to bring in data that is associated with the data already available in the view. A comment view could, for example, use the relationship comment: content to tap into data about the node for each comment. A term view could use the relationship taxonomy: parent term to tap into data from the parent term of each listed term. People comfortable with writing SQL queries will recognize the relationships as joins. You add, edit and delete relationships in the same way as filter ...
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