What is a primary key?

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BlackTom AI · Accepted Answer

The question asks what a primary key is, so we evaluate each option in turn and contrast with the function of a primary key.
Option 1: 'A selected candidate key to uniquely identify records' This aligns with the standard definition: a primary key is chosen from the set of candidate keys and is used to uniquely identify each row in a table. The chosen key must be unique and not null, ensuring each record can be distinguished.
Option 2: 'A key used to create relationships between tables' While a primary key can be used to relate tables via foreign keys in other tables, the primary key itself is not defined by this relational role alone; its core purpose is unique identification. The relationship-building role is more about the foreign key in related tables, not the primary key definition.
Option 3: 'A key that consists of multiple attributes' This is describing a composite key, which is a type of candidate key that happens to involve multiple attributes. However, a primary key does not have to be composite; it can be a single attribute or multiple attributes. Therefore, this statement is not a complete definition and is potentially misleading if interpreted as always.
Option 4: 'A key that can contain null values' This is incorrect because a primary key must not contain null values; nulls would prevent unique identification of records. The rule of primary keys is that they are not nullable to guarantee a unique and present identifier for every row.

What is a primary key?

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A database table usually has a(n) _______________________ that uniquely identifies each row in the table.

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