Performs an aggregate of multiple rows to construct an XML value.
XMLAGG aggregates multiple rows to construct an XML value. During aggregation, for a given collection of rows processed by the function, XML_value_expr is evaluated for every row and the resulting values are ordered according to the ORDER BY clause and concatenated according to the XML value concatenation semantics (same as XMLCONCAT), after eliminating NULLs.
The return value is of XML type. The return value is based on the RETURNING clause. If the clause is not specified, the default is RETURNING SEQUENCE.