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The correct answer is the foreign key. A foreign key is specifically used to establish and enforce a link between the data in two tables. It refers to the primary key in another table, thereby creating a relationship between the two. This relationship is essential for maintaining data integrity and ensuring that records are consistent across the tables involved.
When a foreign key is defined in a table, it ensures that any value in that column must correspond to an existing value in the referenced table's primary key. This allows for relational database management, where data can be normalized and structured logically across multiple tables, reducing redundancy.
The other options serve different purposes:
A primary key uniquely identifies each record within its own table but does not establish a direct connection to another table. A unique key also enforces uniqueness for a column in a single table but is not used to create relationships between tables. A check constraint is used to limit the values that can be stored in a column based on specific conditions, without establishing any relationship to another table.
Understanding how foreign keys work is crucial for anyone managing relational databases, as they enable the necessary interconnections between different datasets.