Keys in DBMS

Keys in DBMS

Introduction

The requirement that every entry, or tuple, in a table be unique is one of the fundamental tenets of the relational database paradigm.This uniqueness ensures accurate data retrieval, eliminates redundancy, and supports data integrity.

While a combination of all attribute values can make each tuple unique, it is often inefficient to rely on all attributes. Instead, a minimal subset of attributes—called keys—can uniquely identify each tuple. These keys form the backbone of relational database design and play a vital role in indexing, constraints, and relationships.

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What Is a Key in DBMS?

A field or set of attributes used to uniquely identify a table entry is called a key in a database management system (DBMS). Keys help enforce entity integrity and establish logical relationships between tables.

To qualify as a key, an attribute or a set of attributes must fulfill two essential conditions:

• Uniqueness: The value of the key cannot be the same in two rows.
• Irreducibility: The key must be minimal. Removing any part of it should violate uniqueness.

Example:A student’s ID acts as a key in a student table because each one is unique. Because they are unique, fields such as SSN, License_Number, or Passport_Number can be used as keys in a PERSON database.

Why Are Keys Necessary in DBMS?

A table may contain thousands of records, and in the absence of unique identifiers, distinguishing between them becomes cumbersome. Keys help solve this issue by:

• Eliminating duplication.
• Establishing entity relationships.
• Supporting data integrity constraints.

They are essential for normalization, query optimization, and relationship mapping in relational databases.

Types of Keys in DBMS

1. Primary Key

The primary key is the main key used to uniquely identify each record in a table. Although multiple attributes may qualify as keys, the most suitable one is chosen as the primary key.

Example:Employee_ID may be chosen as the primary key in an EMPLOYEE database. Other options like Passport_Number or License_Number might also qualify, but one is selected based on design and business requirements.

Purpose:

• Ensures data uniqueness.
• Enables efficient record lookup.
• stops duplicate or null values from appearing in the primary key column.

2. Candidate Key

Any characteristic (or combination of attributes) that meets the requirements to uniquely identify a row is a potential key. Multiple candidate keys may be present in each table, with one of them serving as the primary key.

Properties:

• Unique: No duplicate values.
• Minimal: No subset of the candidate key is itself a key.

Example: SSN, Employee_ID, and License_Number might all be candidate keys in an EMPLOYEE table.

Purpose:

• Helps determine the best key to be used as the primary key.
• Supports schema design and normalization.

3. Super Key

Any collection of characteristics that can be used to uniquely identify a record in a database is called a super key.It can include additional attributes beyond what is necessary.

Example:Both (Employee_ID) and (Employee_ID, Employee_Name) are super keys in the EMPLOYEE database. The only minimal value that could be a candidate key is (Employee_ID).

Purpose:

• Ensures data integrity.
• Helps identify minimal keys (candidate keys) by eliminating unnecessary attributes.

4. Foreign Key

A foreign key is a field (or set of fields) in one table that refers to the primary key in another table. It establishes a link between the child and parent tables.

Conditions:

• The foreign key must either be null or match a primary key in another table.
• The foreign and referenced keys must have the same data types.

For instance, Department_ID may be a foreign key in an EMPLOYEE database that refers to the primary key in the DEPARTMENT table.

Purpose:

• Maintains referential integrity.
• guarantees the validity of the relationships between tables.

5. Alternate Key

Any candidate key that is not chosen as the primary key is considered an alternate key. Depending on how many candidate keys there are, a table may have several alternate keys or none at all.

Example:For instance, PAN_No, if unique, would be an alternate key if Employee_ID were the EMPLOYEE table’s main key.

Purpose:

• Provides flexibility in uniquely identifying records.
• Useful in complex query operations where alternate fields are preferred.

6. Composite Key

A key composed of two or more attributes that collectively uniquely identify a row is called a composite key (also known as a concatenated key).

Example: Assume a worker does several tasks in a variety of projects. A combination of Emp_ID, Proj_ID, and Emp_Role could serve as a composite key.

Purpose:

• Useful in many-to-many relationships.
• Ensures uniqueness when no single attribute is sufficient.

7. Artificial Key

When no natural characteristic is appropriate or when composite keys become too complicated, a system-generated attribute known as an artificial key (or surrogate key) is used to uniquely identify rows.

Example: Instead of using (Emp_ID, Proj_ID, Role) as the composite key, a new field like Record_ID with auto-incremented values may be added as the artificial key.

Purpose:

• Simplifies complex key structures.
• Enhances query performance and schema readability.

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Conclusion

Designing dependable, scalable, and effective databases requires an understanding of the various DBMS key types. Each key type—be it primary, candidate, or foreign—has its specific role in enforcing integrity and supporting database operations.

Choosing the right keys not only ensures that data remains consistent and accessible but also sets the foundation for optimized queries and long-term maintainability.

For more such professional insights into database management systems and software concepts, stay connected with UpdateGadh.

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