There is currently only one national database in the Hungarian health care system: the financial database of the National Health Insurance Fund. It includes all patients' data of state-funded services since 2000, those for pharmaceuticals, health spas and suppliers for patient transport. Any information for scientific research purposes can be obtained from this database. The database for inpatient records is complicated, and it is difficult to make queries. On the basis of his ten-year-experience, the author outlines the typical errors, which hinder a correct query.