This statement defines how the data from disparate variables should be joined. Variable data should have repetitive structures that are related based on some criteria in those structures - the foreign keys that define the relation. Optionally, you can also select specific nodes from the result and modify them to build the resulting document structure.

The <join> statement is comparable to inner joins for databases. Only nodes that have values that match the criteria for all variables are included.

You can obtain outer joins in mashups using XQuery. See Using XQuery for Outer Joins for an example. You can also do self-joins in mashups. See Self-Joins with a Single Dataset for an example.

You must define at least one join condition, as shown in Defining Join Conditions. You can also, optionally, select which nodes to include in the joined items, as shown in Selecting Nodes for Joined Items with <select>. See also Valid Join Operators and Defining Multiple Conditions. See also Working Samples for information on sample mashups that use this statement.