In a relational schema, what is the purpose of a foreign key?

In a relational schema, what is the purpose of a foreign key?

• A. To enforce uniqueness within a table
• B. To provide a cross-table reference to a primary key in another table
• C. To store the entire row in another table
• D. To enforce check constraints

Answer: (B)

The main idea being tested is how a foreign key functions to connect two tables. A foreign key creates a link from a column (or set of columns) in one table to the primary key in another table, establishing a relationship and enforcing referential integrity. This means every value in the foreign key must correspond to an existing row in the referenced table, or be allowed to be null if the design permits. This cross-table reference lets you safely relate data across tables and perform joins to combine related information, such as an orders table that uses a customer ID to connect to a customers table. It’s not about enforcing uniqueness within a single table—that’s what a primary key or a unique constraint does. A foreign key can contain duplicate values and does not by itself store an entire row from the other table; it merely holds the key values that point to the related rows. It also isn’t the mechanism for general check constraints, which enforce other kinds of boolean rules on data.

The main idea being tested is how a foreign key functions to connect two tables. A foreign key creates a link from a column (or set of columns) in one table to the primary key in another table, establishing a relationship and enforcing referential integrity. This means every value in the foreign key must correspond to an existing row in the referenced table, or be allowed to be null if the design permits. This cross-table reference lets you safely relate data across tables and perform joins to combine related information, such as an orders table that uses a customer ID to connect to a customers table.

It’s not about enforcing uniqueness within a single table—that’s what a primary key or a unique constraint does. A foreign key can contain duplicate values and does not by itself store an entire row from the other table; it merely holds the key values that point to the related rows. It also isn’t the mechanism for general check constraints, which enforce other kinds of boolean rules on data.