Question

Explain the concepts of Primary Key, Foreign Key, and Candidate Key.

Hint:

• {Candidate Key} – All possible attributes that can uniquely identify records.
• {Primary Key} – The candidate key selected to uniquely identify records.
• {Foreign Key} – A field that links one table to another table's primary key.

In short: \[ \text{Candidate Key} \rightarrow \text{Primary Key} \rightarrow \text{Foreign Key (link between tables)} \]

Answer 1

Concept: In a relational database, keys are attributes or sets of attributes that help uniquely identify records in a table and establish relationships between tables. Keys play an important role in maintaining data integrity and ensuring efficient data retrieval. 1. Primary Key: A Primary Key is a field or a combination of fields that uniquely identifies each record in a table. It ensures that no two rows have the same value for the primary key. Characteristics of Primary Key:

• Each value must be unique.
• It cannot contain NULL values.
• There can be only one primary key in a table.

Example: In a \texttt{Student} table, the \texttt{Student_ID} can act as the primary key because it uniquely identifies each student. 2. Foreign Key: A Foreign Key is a field in one table that refers to the primary key of another table. It is used to establish a relationship between two tables. Characteristics of Foreign Key:

• It maintains referential integrity between tables.
• It can contain duplicate values.
• It may contain NULL values if the relationship is optional.

Example: In an \texttt{Orders} table, the \texttt{Student_ID} field may refer to the \texttt{Student_ID} in the \texttt{Student} table. 3. Candidate Key: A Candidate Key is an attribute or a set of attributes that can uniquely identify a record in a table. A table may have multiple candidate keys. From the set of candidate keys, one key is selected as the primary key. Characteristics of Candidate Key:

• Must uniquely identify each record.
• Cannot contain duplicate values.
• A table may have multiple candidate keys.

Example: In a \texttt{Student} table, both \texttt{Student_ID} and \texttt{Email} could uniquely identify a student. Thus, both are candidate keys, but one of them is chosen as the primary key.