https://msdn.microsoft.com/library/ms159673

Looking up values from another dataset

The following lookup functions retrieve values from a specified dataset.

Function	Description
Lookup	Returns a value from a dataset for a specified expression.
LookupSet	Returns a set of values from a dataset for a specified expression.
Multilookup Function	Returns the set of first-match values for a set of names from a dataset that contains name/value pairs.

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