In Database Management Systems (DBMS), keys are the backbone of data organization, retrieval, and relationship management. They ensure that databases remain structured, efficient, and free from redundancy. Whether you're a beginner learning SQL or an experienced developer optimizing a database, understanding the different types of keys in DBMS is crucial for designing robust and scalable systems.
Keys serve multiple purposes:
- Uniquely identifying records (e.g., a student ID in a university database).
- Establishing relationships between tables (e.g., linking orders to customers).
- Maintaining data integrity by preventing duplicate or inconsistent entries.
Types of Keys in the Relational Model.
Here is the list of Keys that are present in the Relational Model:
- Primary Key (PK) – Uniquely identifies each row (e.g., Student_ID).
- Foreign Key (FK) – Links tables by referencing a PK (e.g., Order.Customer_ID).
- Candidate Key – A potential PK (e.g., Email if unique).
- Alternate Key – Unused candidate keys (e.g., SSN when ID is PK).
- Composite Key – A PK made of multiple columns (e.g., (Student_ID, Course_ID)).
- Super Key – A set of columns containing a candidate key (e.g., {ID, Name}).
Let's discuss each of them one by one in detail,
1. Primary Key (PK).
A Primary Key (PK) is a column (or set of columns) in a table that uniquely identifies each row. It enforces data integrity by ensuring:
- No duplicate values (all entries must be unique).
- No NULL values (every row must have a PK value).
Key Features
- Uniqueness: Guarantees no two rows have the same PK value. Example: Employee_ID in the Employees table.
- Irreducibility: Cannot remove any column from a composite PK without breaking uniqueness.
- Mandatory: Every table must have exactly one PK (but can have multiple candidate keys).
Example:
CREATE TABLE Students ( Student_ID INT PRIMARY KEY, -- PK column Name VARCHAR(50), Email VARCHAR(100) UNIQUE -- Candidate key );
When to use Primary Key (PK)?
Use the Primary Key when:
- You need to uniquely identify each row in a table, such as assigning a unique EmployeeID to every record in an Employees table.
- You want to enforce data integrity, ensuring no duplicate or NULL values exist in the key column (e.g., ProductID in a Products table).
- Creating relationships between tables, as it serves as the target for Foreign Keys in related tables (e.g., CustomerID in a Customers table referenced by Orders).
- Optimizing query performance, since databases automatically index PKs for faster searches and joins (e.g., filtering orders by OrderID).
- Working with frameworks or ORMs, as most require a PK for CRUD operations (e.g., Django models or SQLAlchemy).
2. Foreign Key (FK).
A Foreign Key (FK) is a column (or set of columns) in a table that references the Primary Key (PK) of another table. It enforces referential integrity, ensuring valid relationships between tables.
Key Properties
- Links Tables: Creates relationships (e.g., Orders.Customer_ID → Customers.Customer_ID).
- Allows NULLs: Optional relationships (unless constrained).
- Supports Cascading Actions: Automatic updates/deletes (e.g., ON DELETE CASCADE).
Example:
CREATE TABLE Orders ( Order_ID INT PRIMARY KEY, Customer_ID INT REFERENCES Customers(Customer_ID) -- FK );
When to use a Foreign Key (FK)?
Use a Foreign Key when:
- You need to link two tables, such as connecting an Orders table to a Customers table via CustomerID.
- You want to enforce referential integrity, ensuring that every record in the child table (e.g., Orders) corresponds to a valid record in the parent table (e.g., Customers).
- You want to automate updates or deletions across related tables, such as deleting all orders when a customer is removed (using ON DELETE CASCADE).
- Modeling relationships like one-to-many (e.g., one customer can have many orders) or many-to-one (e.g., many orders belong to one customer).
3. Candidate Key.
A Candidate Key is a minimal set of columns in a table that can uniquely identify each row without any redundancy. It satisfies two critical properties:
- Uniqueness: No two rows can have identical values in the candidate key columns.
- Irreducibility: No subset of the key can uniquely identify rows (it's "minimal").
Key Features
- A table can have multiple candidate keys.
- One candidate key is chosen as the Primary Key (PK); others become Alternate Keys.
- Candidate Key cannot contain NULL values (to ensure row uniqueness).
Example
In a Students table:
- Student_ID (Unique) → Candidate Key
- Email (Unique) → Candidate Key
- {Name, Birthdate} (If unique) → Composite Candidate Key
Note: All Primary Keys are candidate keys, but not all candidate keys become Primary Keys!
4. Alternate Key.
An Alternate Key is a candidate key that is not selected as the Primary Key (PK) in a table. It retains all properties of a candidate key (uniqueness and minimality) but serves as a backup unique identifier.
Key Features
- Uniqueness: Guarantees no duplicate values in its columns.
- Non-NULL: Cannot contain NULL values (like all candidate keys).
- Secondary Role: Used when the PK is impractical for certain queries.
Example
In a Students table:
- Primary Key (PK): Student_ID
- Alternate Keys: Email or SSN (if both are unique and minimal).
CREATE TABLE Students ( Student_ID INT PRIMARY KEY, Email VARCHAR(100) UNIQUE NOT NULL, -- Alternate Key SSN CHAR(9) UNIQUE NOT NULL -- Alternate Key );
Note: Alternate Keys are implicitly indexed in most databases, just like PKs!
5. Composite Key.
A Composite Key is a type of primary key that consists of two or more columns that together uniquely identify each row in a table. The individual columns may contain duplicate values, but their combination must be unique.
Key Characteristics:
- Multi-Column Uniqueness: The combination of columns must be unique.
- Mandatory Fields: None of the component columns can contain NULL values.
- Relationship Enforcement: Often used in junction tables for many-to-many relationships.
Example:
In an Order_Details table that links orders to products:
CREATE TABLE Order_Details ( OrderID INT NOT NULL, ProductID INT NOT NULL, Quantity INT, PRIMARY KEY (OrderID, ProductID) -- Composite Key );
When to use a Composite Key?
- When you need to uniquely identify relationships between entities
- When a single column isn't sufficient for uniqueness (e.g., flight number + date)
- When you need to uniquely identify records across multiple dimensions
6. Super Key.
A Super Key is any combination of columns in a table that can uniquely identify each row. Unlike candidate keys, super keys may contain extra columns that aren't strictly necessary for uniqueness.
Key Characteristics:
- Uniqueness Guarantee: Must uniquely identify each record.
- Non-Minimal: May include additional columns beyond what's needed for uniqueness.
- Flexible Composition: Can be any size from one column to all columns in the table.
In a Students table with columns:
- Student_ID (unique)
- Email (unique)
- Name
- Birthdate
Possible super keys include:
- {Student_ID}
- {Email}
- {Student_ID, Name}
- {Email, Birthdate}
- {Student_ID, Name, Email, Birthdate}
Conclusion.
Keys ensure data integrity, efficient querying, and logical relationships between tables. From Primary Keys that uniquely identify records to Foreign Keys that link tables, each key type serves a distinct purpose. Candidate Keys offer alternative unique identifiers, while Composite Keys handle complex relationships. Super Keys provide flexibility in uniqueness, and Alternate Keys act as backup identifiers.
Understanding these keys helps in:
✔ Designing optimized and scalable databases
✔ Maintaining data accuracy through constraints
✔ Improving query performance with proper indexing
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