In a relational database management system DBMS, what is a primary key and why is it important for a table?

Introduction / Context:
The concept of a primary key is central to relational database design. Every well designed table should have a primary key that uniquely identifies each row. Without a proper primary key, the database may not be able to enforce entity integrity, and applications can struggle to reference or update specific records reliably. This question checks your understanding of what a primary key really is and how it differs from other columns and indexes.

Given Data / Assumptions:

• We are working in a relational database management system DBMS.
• Each table contains rows and columns representing entities and their attributes.
• The question asks for the best definition of a primary key.
• Primary keys relate to uniqueness and the absence of null values.
• Other answer choices mention indexes, descriptions, and foreign key usage.

Concept / Approach:
A primary key is a special constraint on a column or combination of columns. It ensures that each row can be uniquely identified by enforcing two conditions: all values in the primary key must be unique, and none of them can be null. This supports entity integrity and allows foreign keys from other tables to reference rows unambiguously. Although primary keys are often indexed, not every indexed column is a primary key, and not every descriptive column qualifies as a primary key.

Step-by-Step Solution:
Step 1: Recall the formal definition of a primary key in relational theory and common DBMS implementations. Step 2: Identify the requirement of uniqueness across all rows for the primary key values. Step 3: Recognise that a primary key column or columns cannot contain null values because a missing value cannot uniquely identify a row. Step 4: Compare this definition to the available options and select the one that explicitly mentions unique identification and disallowing null values. Step 5: Confirm that the selected option correctly distinguishes a primary key from simple indexes or descriptive columns.

Verification / Alternative check:
Most DBMS products, such as Oracle, SQL Server, and MySQL, implement primary keys with a primary key constraint. Documentation always emphasises that primary keys enforce uniqueness and prevent null values. They are also commonly used as referenced columns in foreign keys from other tables. Using these facts as a cross check confirms that the correct option must contain both uniqueness and the restriction on nulls.

Why Other Options Are Wrong:
Option B is incorrect because it describes an indexed column that may contain duplicate values; uniqueness is not guaranteed, so it cannot be a primary key. Option C is wrong because a descriptive column such as a name does not automatically guarantee uniqueness or non null values. Option D contradicts the definition of a primary key by allowing nulls and uncertainty about uniqueness. Option E misrepresents foreign keys; columns used as foreign keys reference a primary key or unique key in another table and may contain duplicates, but that does not make them primary keys themselves.

Common Pitfalls:
Learners sometimes assume that any column with an index is a primary key, which is not true. Another frequent mistake is to choose a descriptive attribute, such as a name, as a primary key even when duplicate names are possible. Some designs accidentally allow null values in key columns, undermining integrity. Always remember that a primary key must uniquely and reliably identify each row without any missing values.

Final Answer:
In a DBMS, a primary key is a column or set of columns whose values uniquely identify each row in a table and that does not allow null values..