early-termination conditions elements collation model queries path notation XMLAgg( ) function XMLElement( ) function entities entity-relationship models, sample database equality conditions equality operator (=) equi-joins 2nd equiheight buckets equivalence classes, regular expressions equiwidth buckets errors aggregate functions compound queries DECODE function divide by zero, avoiding GROUP BY clause HAVING clause inline views 2nd multiple-row subqueries updating columns evaluation, WHERE clause conditions existing cells, updating EXISTS operator compared to DISTINCT correlated subqueries existsNode( ) member function EXPLAIN PLAN, group operations expressions 2nd 3rd aggregate functions 2nd aggregate, GROUP BY clause ALL keyword CASE advantages controlling state optional searched selective function execution simple transforming result sets UPDATE statement conditions DECODE function DISTINCT keyword GROUP BY clause nested group operations nonaggregate, GROUP BY clause NULL values regular backreferences collation elements equivalence classes functions named character classes Perl usage syntax TABLE, querying collections testing XPath, inspecting XML documents Extensible Markup Language [See XML] extensions EXTRACT function extract( ) member function extractValue( ) member function