1. A computer implemented method for data conversion, the method comprising: receiving from an application a conversion request for data stored in a database; invoking the database to export the data into a conversion source file by bypassing an application programming interface between the application and the database, wherein data associated with processing the conversion request passes between the database and the application without passing the application programming interface; obtaining, from the application, a set of rules for generating converted data, at least one rule of the set of rules comprising a first part having at least one query specifying at least one subset of the data to convert, a second part having at least one mathematical expression that defines how the at least one subset of the data matching the at least one query is converted, and a third part having at least one value table including mapping rules for mapping between old values and new values, the set of rules being defined in a relation table that reflects the relationship between the at least one query stored in a query table, the at least one mathematical expression stored in an expression table, the at least one value table, and the order in which the set of rules are applied to the data, wherein the relation table includes a first foreign key referencing the query table, a second foreign key referencing the expression table, and a third foreign key referencing the at least one value table; converting the data of the conversion source file according to the set of rules; storing the converted data in a conversion target file, wherein the converting step comprises: checking to see whether the at least one subset of the data matches the at least one query; and applying the at least one mathematical expression and the at least one value table, related to the at least one query, to the at least one subset of the data; storing an indication of an error in an error log file when an error is detected during conversion of the data to the conversion target file; and providing the error log file to the application for error handling.