Which of the following is not a feature of a Foreign Key?

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Question: Which of the following is not a feature of a Foreign Key?

Option 1: 'It is a unique column in a relation.' This statement is not a feature of a foreign key. A foreign key can contain duplicate values across rows because it may reference the same value in the parent table for multiple child rows. For example, many orders can reference the same customer ID. So claiming that a foreign key must be unique is incorrect, making this option a not-a-feature statement.

Option 2: 'A Foreign key cannot contain duplicate values and null values.' This is also incorrect as a general rule. While a foreign key may be defined as NOT NULL in some designs, it is common for a FK to allow duplicates (many rows can refer to the same parent key) and to be nullable in cases where the relationship is optional. Therefore, the blanket claim that it cannot contain duplicates or nulls is false.

Option 3: 'Foreign keys by themselves do not constrain data.' This is misleading. A foreign key constraint enforces referential integrity, preventing child values that do not exist in the parent table. In other words, FK constraints do constrain data by ensuring that references are valid. Saying they do not constrain data is inaccurate.

Option 4: 'It can constrain data on its own.' This is generally true in the sense that a foreign key constraint enforces referential integrity by itself, provided the constraint is properly defined and the referenced table/column exists. It contributes to constraining data without requiring additional constraints.

Summary: Options 2 and 3 contain incorrect claims about the nature and behavior of foreign keys, while option 1 asserts an attribute (uniqueness) that does not apply to typical foreign keys. Option 4 correctly reflects that a foreign key constraint can constrain data. The explanation clarifies why each statement is true or false in the context of relational database design.

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Which of the following is not a feature of a Foreign Key?

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