Oracle Database SQL — Question 136

Which four statements are true about constraints on Oracle tables? (Choose four.)

Answer options

• A. A PRIMARY KEY constraint can be added after a table has been created and populated.
• B. A FOREIGN KEY column can contain NULLs.
• C. A CHECK constraint can refer to values in other rows.
• D. A NOT NULL constraint can be defined at the table level.
• E. A UNIQUE constraint can use a pre-existing index on the constrained column or columns.
• F. A UNIQUE constraint permits NULLs.
• G. A column can have only one CHECK constraint.

Correct answer: A, B, E, F

Explanation

The correct answers A, B, E, and F are valid statements about Oracle constraints. A PRIMARY KEY can indeed be added after table population, FOREIGN KEYs can accept NULL values, UNIQUE constraints can leverage existing indexes, and UNIQUE constraints allow multiple NULLs. However, options C, D, and G are incorrect because a CHECK constraint cannot reference other rows, NOT NULL constraints must be defined at the column level, and a column can have multiple CHECK constraints.