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The ability to evaluate the logical correctness of SQL queries is a vital feature of an intelligent tutoring system that provides meaningful feedback and guidance to students that learn SQL. This process relies on comparing the student's query with a reference query provided by the instructor. Converting the SQL source code into forms of XML representation of the query turns out to be an effective way to process and analyze the semantics of SQL queries. Queries of arbitrary complexity can be compared effectively through XML, using two forms of representation: structural representation and full content representation. Further, one can apply query transformation patterns that convert a query into a logically equivalent one, having a different structure, but still equivalent to the initial query. This paper focuses on the XML based technique to compare and to transform SQL queries in order to determine their logical compliance with a predetermined reference.