join_key = 'join_key'

Marks join keys.

Role required to establish a relation between two Placeholder, the abstract representation of the DataFrame, by using the join() method (see Data model). Please refer to the chapter Data model for details.

The content of this column is allowed to contain NULL values. But beware, columns with NULL in their join keys won’t be matched to anything, not even to NULL in other join keys.

columns of this role will not be handled by the feature learning algorithm.