https://www.geeksforgeeks.org/constraints-on-relational-database-model/amp/

https://en.m.wikipedia.org/wiki/Referential_integrity

C can take either null values or some value of D. The only restriction is that C should not take any value which is not equal to some row value of D.

https://en.m.wikipedia.org/wiki/Referential_integrity

C can take either null values or some value of D. The only restriction is that C should not take any value which is not equal to some row value of D.