Yes, a NULL value can be assigned to that student's preference field. In relational databases, NULL represents unknown or missing data. Since Roll no 24 has not provided any preference, NULL appropriately indicates that the value is unknown or not available. However, if a NOT NULL constraint is applied on the Preference column, then NULL cannot be assigned.

The property violated is Entity Integrity. This principle states that no attribute participating in the primary key of a relation should accept NULL values, and the primary key must uniquely identify each tuple. Having two tuples with the same Roll_no (17) violates the uniqueness requirement.

To prevent such violation, we can use Primary Key constraint on the Roll_no column. A primary key ensures that each roll number is unique and does not allow duplicate values. Alternatively, a Unique constraint can also be applied to ensure all values in the Roll_no column are distinct.

No, it is not possible to have this tuple in the Sports Preferences relation if Roll_no is defined as the primary key. According to Entity Integrity, the primary key cannot contain NULL values. The tuple (NULL, Kabaddi) violates this rule because the Roll_no attribute is NULL. If we need to store sports that are available but not chosen by any student, a separate table for sports should be created rather than inserting tuples with NULL roll numbers in the preferences relation.