Consider the following relation definition:

STUDENT(

Student_Number: integer NOT NULL

Name: variable length character string length 20 NOT NULL)

Primary Key Student_Number

HOUSING(

Housing_ID: integer NOT NULL

Student_Number: integer NOT NULL

Building: variable length character string length 25 NOT NULL)

Primary Key Housing_ID

Foreign Key Student_Number References

STUDENT(Student_Number)

ON DELETE NO CHECK

ON UPDATE

Which integrity constraint is violated in this relation definition?