Understanding database keys is foundational to grasping how relational databases work. Database keys help to identify records uniquely, establish relationships between tables, ensure data integrity, and facilitate data manipulation. They serve as the cornerstone for complex operations like joins, updates, and deletes. In this article, we will delve deep into the types of database keys, their properties, uses, and practical examples.
1. Primary Key
The primary key uniquely identifies each record in a table. It cannot contain NULL values, and each table can have only one primary key. The primary key is usually set during table creation and can be composed of one or multiple columns.
Example
CREATE TABLE Students (
StudentID INT PRIMARY KEY,
FirstName VARCHAR(50),
LastName VARCHAR(50)
);
2. Foreign Key
A foreign key is a column or a set of columns that is used to establish a relationship between the data in two tables. It is related to the primary key in another table and ensures referential integrity.
Example
CREATE TABLE Enrollments (
EnrollmentID INT PRIMARY KEY,
StudentID INT,
CourseID INT,
FOREIGN KEY (StudentID) REFERENCES Students(StudentID)
);
3. Unique Key
A unique key is a key that ensures all the values in a column are different. Unlike the primary key, a table can have multiple unique keys, and the unique key column can have NULL values.
Example
CREATE TABLE Departments (
DepartmentID INT PRIMARY KEY,
DepartmentCode INT UNIQUE
);
4. Composite Key
A composite key is a key that consists of more than one column to
uniquely identify each record. It is used when a single column is not
sufficient to uniquely identify records.
Example
CREATE TABLE OrderDetails (
OrderID INT,
ProductID INT,
PRIMARY KEY (OrderID, ProductID)
);
5. Candidate Key
A candidate key is a key that can uniquely identify records in a table. A table may have multiple candidate keys, but one of these is chosen as the primary key. Candidate keys are essentially unique keys that can be primary if chosen.
Example
CREATE TABLE Books (
ISBN INT PRIMARY KEY,
BookCode INT UNIQUE
);
6. Super Key
A super key is a set of one or more columns that can be used to uniquely identify records within a table. The difference between a super key and a candidate key is that a super key can have additional columns that are not strictly necessary for unique identification.
Example
In the Students table, a super key could be (StudentID, FirstName).
7. Alternate Key
Alternate keys are keys that are not selected as the primary key but
could be if needed. They are essentially candidate keys that are not
chosen as the primary key.
Example
In the Books table, BookCode could be an alternate key.
8. Natural Key and Surrogate Key
Natural Key: A key that has a logical relationship to the attributes within that row.
Surrogate Key: A key that has no logical meaning and is usually auto-generated by the system.
Example
In the Employees table, an EmployeeID could be a surrogate key, whereas an Employee Social Security Number could be a natural key.
Data Integrity: Keys enforce data integrity by ensuring that the data within the database is accurate and consistent.
Efficiency: They improve query performance by enabling better indexing strategies.
Relationships: Keys define the relationships between tables, making it easier to join tables and perform complex queries.
Understanding database keys is vital for anyone dealing with databases. They serve to enforce integrity, improve query performance, and define relationships. A thorough grasp of database keys is crucial for effective data manipulation and integrity in relational databases.