1. A merging apparatus comprising: a memory; and a processor coupled to the memory, wherein the memory includes a first database that store therein, as records, each of attribute values that is classified under one or a plurality of attributes and that is associated with each other, in which a plurality of attributes including a key attribute that is set in advance as a key for merging is set; a second database that store therein, as records, each of the attribute values, in which one or a plurality of attributes excluding the key attribute is set; and the processor is configured to execute a process comprising checking the attribute that is set in the first database against the attribute that is set in the second database and extracting a common attribute that is set, in common, in both the first database and the second database; and merging the first database with the second database by determining whether attribute values classified under the common attribute that is extracted in the extracting match, wherein the merging, when a plurality of common attributes are extracted, merges the first database with the second database by using a common attribute which is selected from the plurality of common attributes, the common attribute is selected based on first record numbers and second record numbers, each of the first record numbers corresponding to one of the plurality of attributes stored in the first database and being the number of records in which an attribute value classified under the one of the plurality of attributes does not match any of the attribute values contained in the other records and classified under the one of the plurality of attributes, each of the second record numbers corresponding to one of the plurality of attributes stored in the first database and being the number of records containing the attribute value classified under the one of the plurality of attributes.