Oracle Database Administration I — Question 155

Which two statements are true about constraints? (Choose two.)

Answer options

• A. A UNIQUE constraint can use a pre-existing index on the constrained column or columns
• B. A FOREIGN KEY column can contain NULLs.
• C. A PRIMARY KEY constraint can only be added to an empty table.
• D. A column can have only one CHECK constraint.
• E. A CHECK constraint can refer to values in other rows.

Correct answer: A, B

Explanation

The correct answers are A and B. Option A is correct because a UNIQUE constraint can indeed utilize an existing index on the specified columns. Option B is also correct since a FOREIGN KEY can allow NULL values. Options C, D, and E are incorrect because a PRIMARY KEY can be added to a table with existing data, a column can have multiple CHECK constraints, and CHECK constraints cannot refer to other rows.