There is no reason why a Primary key column cannot be a Foreign Key
Table #1 Persons
Id (Primary) Name
1 - Raj
2 - Majid
3 - Joseph
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Table #2 Projects - Reference Id column of Persons Table
PersonId (Primary and Foreign) ProjectId
1 - Data Referencing
2 - E Store
3 - Sales Report
Now in the projects table I want one Person to work only on one project hence I made it a Primary Key as well as Foreign Key as I need two tasks
1. Unique Values - One Person on one project only
2. Constraint - so that the PersonId must be from Person Table