1. A database management system that reduces processing costs for ensuring uniqueness of a target column in a database, the system comprising: an external storage system that stores the database that includes a plurality of data tables; a server that includes: a network interface communicatively coupled to the external storage system, a memory that stores comparison reference values, and a processor communicatively coupled to the network interface and the memory; wherein the processor of the server: receives, using the network interface, a new data table input to the database of the external storage system; executes unique constraint determination processing to generate a unique constraint, wherein the unique constraint determination processing includes first stage determination processing and second stage determination processing that is executed when a first determination result obtained in the first stage determination processing is true; and stores a record from the new data table in the database using the unique constraint in the target column; wherein: the first stage determination processing includes determining the first determination result that indicates whether any of comparison-subject values in the new data table is included in a column value range in a stored data table of the database, the column value range having maximum and minimum values respectively corresponding to maximum and minimum values of the comparison reference values, the second stage determination processing includes determining a second determination result that indicates whether all in-range comparison-subject values in the new data table are different from all the comparison reference values, the in-range comparison-subject value being the comparison-subject values included in the column value range, and when the second determination result is true, the processor stores the new data table in a second database area different from a first database area storing the stored data table in the database.