When validating database mappings in Corticon Studio, the validation message listed below is observed:
Why does this warning message show up ?

The  DB validation message appears because a column cannot be used in a join expression and mapped to an attribute at the same time.
For Entity WildCardTest, the sfdc_guid column is being mapped to both the sfdc_guid attribute and is used in the join expression for the territoryAssignment association. 
Corticon only expects one such mapping.

User should not have 2 columns in the DB, with the same data, with one for the join and one for the attribute mapping.
The proper solution would be not to model the same column as an attribute and an association.
Usually a Foreign Key value would not be used in an explicit rule. So it would not need to be mapped to an attribute.
Foreign Key comparisons/modifications are abstracted into association.
Remove the attribute from the vocabulary, and only use it for the join expression to the associated entity. 

If a reference to the attribute value is required for rule processing, simply traverse to the associated entity and reference the value.
Example:
A person with multiple jobs
Person.jobTitle = Job.title is the join
Person.jobTitle would not be in the vocabulary, only in the join
In rules, use Person.job.title = ‘xxxxx’