When you create the source-to-lookup relationship, you select the fields on which you want to join the source and lookup tables.
The fields included in a join condition must have compatible datatypes, precisions, and scales. For example, you can join an INT field to a DECIMAL field, but not to a DATETIME or VARCHAR field. Data Validation Option supports joins on fields with the following datatypes: datetime, numeric, and string. Joins are not allowed on binary/other datatypes.