Multiple choice technology

Which statements are true regarding constraints?

  1. A foreign key cannot contain NULL values.

  2. A column with the UNIQUE constraint can contain NULL values.

  3. A constraint is enforced only for the INSERT operation on a table.

  4. A constraint can be disabled even if the constraint column contains data.
Reveal answer Fill a bubble to check yourself
B,D Correct answer
Explanation

To answer this question, the user needs to know about the concept of constraints in databases.

A. This statement is true. If a foreign key column contains null values, it cannot reference any primary key values in the referenced table, resulting in a violation of referential integrity. Hence, a foreign key cannot contain NULL values.

B. This statement is true. A column with the UNIQUE constraint can contain NULL values because the UNIQUE constraint only enforces the uniqueness of non-null values. However, only one null value can be present in the column as multiple null values would not be unique.

C. This statement is false. A constraint is enforced for every operation on a table, including INSERT, UPDATE, and DELETE operations.

D. This statement is true. A constraint can be disabled using the ALTER TABLE statement even if the constraint column contains data. However, disabling a constraint can result in data inconsistency, so it should be done with caution.

Therefore, options A, B, and D are true, and option C is false.

The Answer is: D

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