Which of the following is a characteristic of a foreign key?

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BlackTom AI · Accepted Answer

Question restatement: Which of the following is a characteristic of a foreign key?

Option 1: 'Must be unique' — This is incorrect. A foreign key is not required to be unique. In a child table, multiple rows can reference the same row in the parent table, so the foreign key values can repeat.

Option 2: 'Cannot be null' — This is not necessarily true. A foreign key column can be nullable, meaning a row may exist without a related row in the parent table, unless a NOT NULL constraint is explicitly applied.

Option 3: 'Is always a composite key' — This is false. A foreign key can consist of a single column or multiple columns (composite), but it is not required to be composite in all cases.

Option 4: 'Can contain duplicate values' — This is correct. Multiple child rows can reference the same parent row, so foreign key values may duplicate across rows in the child table. This reflects the many-to-one relationship from the child to the parent.

Additional clarification: The essential role of a foreign key is to enforce referential integrity by tying a column (or set of columns) in the child table to the primary key (or a candidate key) in the parent table. However, the exact constraints about uniqueness and nullability depend on how the key is defined and constrained in the schema.

Which of the following is a characteristic of a foreign key?

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