SQL4X, a powerful language for simultaneously querying 
both relational and XML databases, is presented. SQL4X queries can 
create both relations and XML documents as results. Thus, SQL4X can 
be thought of as an integration language. In order to allow easy inte- 
gration of XML documents with varied structures, SQL4X uses exible 
semantics. SQL4X is also a powerful query language. It can express quan- 
tication, negation, aggregation, grouping and path expressions. 
Datalog4x and Tree-Datalog4x , extensions of Datalog, are dened as el- 
egant abstract models for SQL4X queries. Query containment is char- 
acterized for many common classes of SQL4X queries; for example, the 
class of conjunctive Datalog4x queries and the class of unions of such 
queries. Equivalence of Datalog4x queries under bag-set semantics is also 
characterized. A suÆcient condition for containment of Tree-Datalog4x 
queries is presented. This condition is shown to be complete for a large 
class of common queries.