When it is necessary to join a table containing historical building records with a layer of building footprints in a geodatabase, what type of association is appropriate?

The most appropriate association in this scenario is a Relationship class. A Relationship class is specifically designed in a geodatabase to define a relationship between two tables where one table can hold multiple records related to a single record in another table. This is particularly useful when dealing with historical building records, where you may have several records corresponding to various instances or modifications of a particular building footprint.

Using a Relationship class allows you to maintain these complex relationships while ensuring data integrity and enabling efficient querying. This is particularly important since historical records often include multiple entries for different time periods or modifications of a single building, making a one-to-many relationship necessary.

A simple join might not effectively represent the complex relationships inherent in historical records, as it typically works best with one-to-one or many-to-one relationships. While a relate could be an option, it does not provide the same level of integration and data management as a Relationship class within a geodatabase context. A spatial join, on the other hand, is used to combine two spatial datasets based on their location rather than their attributes, which is not applicable for the association between historical records and building footprints in this case.