What does `PRIMARY KEY` in a table ensure?

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BlackTom AI · Accepted Answer

When evaluating what PRIMARY KEY enforces in a relational table, we start by unpacking its core constraints and typical behavior.
Option 1: 'Columns are of the same data' is not a property of a primary key. A primary key can consist of one column or multiple columns, and those columns can be of different data types or even include a composite key. The statement about uniform data type across columns does not capture the PK concept.
Option 2: 'Columns are auto-incremented' describes a common pattern for surrogate keys, but it is not what a PRIMARY KEY guarantees by itself. An auto-increment property is a separate mechanism (often defined on a specific column) and is not an inherent requirement of a primary key, which can be natural keys as well.
Option 3: 'Columns contain unique, non-null values' aligns with the fundamental role of a PRIMARY KEY: it uniquely identifies each row and does not allow nulls. This ensures row identity and data integrity, which is why the PK enforces both uniqueness and non-nullability.
Option 4: 'Columns are indexed' is a closely related consequence, since most database systems create a unique index to enforce the primary key constraint. However, the wording implies indexing as an inherent property rather than the essential rules of uniqueness and non-nullability. While an index often exists for a PK, the critical, defining feature is the unique, non-null constraint rather than the mere presence of an index.
In summary, the defining requirement of a PRIMARY KEY is that the designated columns uniquely identify each row and never contain NULL values, which makes option 3 the core correct description, while the other options describe related ideas or common implementations but do not capture the essential constraint.

What does `PRIMARY KEY` in a table ensure?

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Question at position 11 In ERDs, the primary key isAn attribute that cannot be modified after it is setAn attribute that uniquely identifies each record in a database tableA graphical symbol indicating the connection between entitiesA set of optional attributes linked to an entity

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

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