GATE CS 2017 Set 2 Q19 — Integrity Constraints

To find the number of additional records deleted from table $T1$ when we delete the record $\langle 3, 8 \rangle$, let us trace the referential integrity constraints step-by-step.

1. Initial Deletion

We delete the record $\langle 3, 8 \rangle$ from table $T1$.

• This removes the primary key value $P = 3$ from $T1$.

2. Check References to $P$ in $T1$

The attribute $S$ in table $T2$ is a foreign key referencing $P$ in $T1$ with the rule ON DELETE SET NULL.

• We look for records in $T2$ where $S = 3$.
• The record $\langle 8, 3 \rangle$ in $T2$ has $S = 3$.
• Applying the ON DELETE SET NULL action, this record is updated to $\langle 8, \text{NULL} \rangle$.
• Note: This is an update operation on $T2$, not a deletion. No records are deleted from $T2$.

3. Check References to $R$ in $T2$

The attribute $Q$ in table $T1$ is a foreign key referencing $R$ in $T2$ with the rule ON DELETE CASCADE.

• Since no records were deleted from $T2$, this cascade action is not triggered.
• No further records in $T1$ are affected.

Conclusion

No additional records need to be deleted from table $T1$.

Therefore, the number of additional records that need to be deleted from $T1$ is 0.