Muenchian grouping

Muenchian grouping is a style of grouping used in XSL Transformations that identifies keys in the results and then queries all nodes with that key. This differs from the common alternative whereby each node is checked against the previous node to determine if the key is different (this would indicate the beginning of a new group). This method also requires that the XML is already ordered by the key. In both cases the key can take the form of an attribute or element.

The unique identifier is referred to as a key because of the use of the 'key' function to identify and track the group variable.

The technique should become obsolete in XSLT 2.0 which introduces the new for-each-group tag.