Data query method, data query system and computer readable and writable recording medium

A relational database query system for a relational database is provided, wherein the relational database comprises several relational tables including at least one constant table storing several unique data. The relational database query system comprises a query receiving module for receiving an extensible markup language query (XML query); at least a constant mapping tree respectively corresponds to one of the constant tables, wherein the constant mapping tree comprises several nodes respectively with node numbers respectively corresponding to the unique data in the constant table; a query generator for mapping the XML query to be a structured query language (SQL) according to the constant mapping tree; a query module for querying the relational database with the SQL.