In the relational model, keys play a crucial role in identifying and establishing relationships between data stored in tables. The types of keys in relational model ensure data accuracy, uniqueness, and integrity within a database. Each key has a specific purpose, from uniquely identifying records to linking tables through relationships.
Understanding the various types of keys is essential for efficient database design and query optimization.
What Are Keys in Relational Model?
Keys are attributes or combinations of attributes that uniquely identify a record (row) in a table (relation). They are critical in maintaining data consistency and enforcing relational integrity.
Types of Keys in Relational Model
1. Primary Key
A primary key is an attribute or a set of attributes that uniquely identifies a tuple (row) in a table.
Characteristics:
- Unique: No two rows can have the same value for the primary key.
- Non-Null: Primary keys cannot contain null values.
- Minimal: Only the essential attributes are included.
Example:
In a Student table, StudentID can serve as the primary key.
| StudentID | Name | Age |
|---|---|---|
| 101 | John Doe | 20 |
| 102 | Jane Doe | 22 |
2. Candidate Key
A candidate key is a set of attributes that can potentially serve as the primary key.
Characteristics:
- Uniquely identifies each record.
- Contains no unnecessary attributes.
- A table can have multiple candidate keys, but only one is chosen as the primary key.
Example:
In the Student table, both StudentID and Email could be candidate keys.
3. Super Key
A super key is a set of attributes that can uniquely identify a tuple in a relation. It may include redundant attributes, unlike a candidate key.
Characteristics:
- Includes all candidate keys.
- May consist of additional attributes beyond what is necessary for uniqueness.
Example:
If StudentID is a primary key, {StudentID, Name} is a super key.
4. Foreign Key
A foreign key is an attribute in one table that references the primary key of another table. It establishes a relationship between the two tables.
Characteristics:
- Ensures referential integrity.
- Allows establishing many-to-one or many-to-many relationships.
Example:
In an Enrollment table, the StudentID column can be a foreign key referencing the StudentID in the Student table.
5. Composite Key
A composite key consists of two or more attributes that uniquely identify a tuple in a relation.
Characteristics:
- Used when a single attribute is insufficient to guarantee uniqueness.
- Combines attributes to create a unique identifier.
Example:
In an Order table, OrderID and ProductID together could form a composite key.
6. Alternate Key
An alternate key is a candidate key that is not chosen as the primary key.
Characteristics:
- Still uniquely identifies a tuple.
- Serves as a backup for the primary key.
Example:
If StudentID is the primary key, Email could be an alternate key.
7. Unique Key
A unique key is similar to a primary key but can accept a single null value.
Characteristics:
- Ensures all values in the column are unique.
- Used to enforce uniqueness on attributes that are not the primary key.
Example:
In a User table, Email can be a unique key to prevent duplicate email addresses.
Differences Between Keys in Relational Model
| Key Type | Purpose | Allows Null? | Example |
|---|---|---|---|
| Primary Key | Uniquely identifies records. | No | StudentID in Student table |
| Foreign Key | Links tables through relationships. | Yes | StudentID in Enrollment |
| Candidate Key | Potential primary keys. | No | Email, PhoneNumber |
| Composite Key | Combines attributes for uniqueness. | No | OrderID, ProductID |
| Unique Key | Ensures attribute uniqueness. | Yes (1 allowed) | Email in User table |
Importance of Keys in Relational Model
1. Ensures Data Integrity
Keys enforce rules to ensure data accuracy and consistency across tables.
2. Facilitates Relationships
Foreign keys establish and maintain relationships between tables.
3. Prevents Redundancy
Primary keys and unique keys avoid duplicate data entries.
4. Optimizes Queries
Keys help in indexing, making data retrieval faster and more efficient.
Practical Applications of Keys
- E-commerce Systems:
- Primary keys for
ProductID. - Foreign keys for linking
OrdersandCustomers.
- Primary keys for
- Banking Systems:
- Composite keys for identifying account transactions.
- Unique keys for customer identification.
- Healthcare Databases:
- Primary keys for
PatientID. - Foreign keys for linking patients and treatments.
- Primary keys for
FAQs: Types of Keys in Relational Model
1. What is the difference between a primary key and a unique key?
A primary key does not allow null values, while a unique key permits one null value. Both ensure uniqueness in their columns.
2. Can a table have multiple foreign keys?
Yes, a table can have multiple foreign keys, each referencing different primary keys in other tables.
3. Why are composite keys used?
Composite keys are used when no single attribute can uniquely identify a record, requiring a combination of attributes for uniqueness.
4. What is the significance of a super key?
A super key ensures that a record can be uniquely identified, but it may include unnecessary attributes.
5. Can a candidate key be a primary key?
Yes, one of the candidate keys is chosen as the primary key for a table.