SQL Transparent Hierarchical Processing of Relational, XML and IMS Data
The SQL-92 standard introduced the LEFT Outer Join operation which offers a powerful alternative to standard relational processing to perform full hierarchical processing naturally and inherently. This enables SQL to transparently integrate relational data with XML, IMS and other forms of legacy hierarchical data. This can also allow the so far overlooked integration of IMS and legacy data with the Internet and XML, and visa versa.

