XML is emerging as one of the dominant data formats for data processing on the Internet. To query XML data, query languages like XQL, Lorel, XML-QL, or XML-GL have been proposed. In this paper, we study how XML data can be stored and queried using a standard relational database system. For this purpose, we present alternative mapping schemes to store XML data in a relational database and discuss how XML-QL queries can be translated into SQL queries for every mapping scheme. We present the results of comprehen- sive performance experiments that analyze the tradeoffs of the alternative mapping schemes in terms of database size, query performance and update performance. % The results show clearly that a variant of a binary mapping scheme % shows the overall best performance While our discussion is focussed on XML and XML-QL, the results of this paper are relevant for most semi-struct- ured data models and most query languages for semi-structured data.