7. A computer-readable storage medium containing a program which, when executed, performs an operation, comprising: mapping a first reference value from a first set of reference data to a first ontology value in an ontology, wherein the first reference value characterizes one or more data values in a first database; mapping a second reference value from a second set of reference data to a second ontology value in the ontology, wherein the second reference value characterizes one or more data values in a second database; determining a first association between the first ontology value and the second ontology value; and based on the determined first association, and by use of one or more computer processors, determining a second association between the first value from the first set of reference data and the second value from the second set of reference data.
10. The computer-readable storage medium of claim 7 , the operation further comprising: identifying the first set of reference data, containing a first plurality of reference values, comprising: for each table in a first database: determining a size of the table; determining whether the table is a reference table based on the determined size for the table; and if the table is determined to be a reference data table, adding the table to the first set of reference data.