Which of the following operations will NEVER violate the foreign key constraint?
Understanding Foreign Key Constraints
A foreign key (FK) constraint ensures referential integrity between two tables. In this case, the attribute E in relation S must reference a valid, existing value of attribute A in relation R. Attribute A is the primary key (PK) for R, meaning each A value is unique within R.
The constraint is specifically: Any value in S.E must exist in R.A.
Analyzing Operations for FK Violations
We need to identify operations that will *never* violate this constraint.
Operation 1: Inserting records into relation r
Inserting a record into r involves adding a pair (A, B). Since A is the primary key of r, a new A value is added (or potentially an existing one updated, depending on constraints). This operation does not involve the S relation or the E attribute. Therefore, it cannot violate the constraint that requires S.E to reference R.A.
- Conclusion: Does NOT violate the FK constraint.
Operation 2: Deleting records from relation s
Deleting a record from s removes a pair (E, C). The FK constraint checks if the value of E exists in R.A *before or during insertion/update* into s. Deleting a row from s removes a value from E, but it does not remove the corresponding value from R.A. The constraint remains satisfied because the remaining rows in s (if any) still reference existing A values in r, and no new invalid references are created.
- Conclusion: Does NOT violate the FK constraint.
Operation 3: Deleting records from relation r
Deleting a record from r means removing a pair (A, B). If the deleted A value exists in the E column of any record in s, the FK constraint will be violated. This is because there would be records in s referencing a non-existent A value in r (unless specific rules like CASCADE DELETE or SET NULL are implemented, but standard FK constraints would be violated).
- Conclusion: CAN violate the FK constraint.
Operation 4: Inserting records into relation s
Inserting a record into s requires providing a value for E. If the provided value for E does not exist in the primary key column A of relation r, the FK constraint is immediately violated.
- Conclusion: CAN violate the FK constraint.
Final Determination
Based on the analysis, the operations that will NEVER violate the foreign key constraint (S.E references R.A) are:
- Inserting records into relation r.
- Deleting records from relation s.
Important Questions from Integrity Constraints
- Consider the following two relations, named Customer and Person, in a database:
Person (
aadhaar CHAR(12) PRIMARY KEY,
name VARCHAR(32));
Customer (
name VARCHAR(32),
email VARCHAR(32) PRIMARY KEY,
phone CHAR(10),
aadhaar CHAR(12),
FOREIGN KEY (aadhaar) REFERENCES Person(aadhaar));
Which of the following statements is/are correct?
