In the vast world of databases, keys play a crucial role in organizing and maintaining data integrity. They act as special identifiers, ensuring the unique identification of records and establishing relationships between different tables. In this article, we will explore the various types of keys used in Database Management Systems (DBMS) and understand their distinct functions with simple examples.
- Primary Key: The Superstar Identifier The primary key is the backbone of a database table. It is a unique identifier for each record, ensuring there are no duplicates in the table. Consider a table named “Students” where the primary key could be “Student ID.” This key guarantees that each student has a distinct ID, like 101, 102, and so forth.
- Foreign Key: For Linking and Relationships The foreign key creates connections between different tables in a database. It references the primary key of another table and helps maintain relationships. Let’s say we have two tables: “Students” and “Courses.” To connect them, the “Students” table will contain a foreign key called “Course ID,” referencing the “Course ID” primary key in the “Courses” table. This way, we can link each student to the course they are enrolled in.
- Unique Key: Preventing Duplicates The unique key ensures the uniqueness of values in a column, acting as a secondary key. While a table may have multiple unique keys, they serve to prevent duplicate entries. For example, in a table called “Email Subscribers,” we can create a unique key for the “Email Address” column, ensuring no two subscribers share the same email address.
- Composite Key: The Combination Lock As the name suggests, a composite key is formed by combining two or more columns to create a unique identifier. Picture it as a combination lock, where each column contributes to the uniqueness. In an “Orders” table, a composite key could consist of “Order ID” and “Product ID,” ensuring that no two orders have the same combination of these IDs.
- Candidate Key: Potential Primary Key A candidate key is a potential contender for the primary key. Each candidate key can uniquely identify records, but ultimately only one will be chosen as the primary key. For example, in an “Employees” table, we may have two candidate keys: “Employee ID” and “Social Security Number.” The decision will be made to select one of them as the primary key.
- Surrogate Key: The Artificial Identifier When a natural primary key is unavailable or impractical, a surrogate key comes to the rescue. It is an artificially generated key, often an auto-incremented number, assigned to each record. Imagine a table named “Customers” where a surrogate key like “Customer ID” is used, automatically generated as new customers are added to the table.
How to Remember these keys?
- Primary Key: The main identifier for each record in a table. Unique and helps prevent duplicates. Each table has only one primary key.
- Foreign Key: Creates a connection between two tables. It references the primary key of another table and helps maintain relationships between them.
- Unique Key: Ensures the uniqueness of values in a column, like a secondary key to avoid duplicates. A table can have multiple unique keys.
- Composite Key: Made up of two or more columns working together as a unique identifier, like a combination lock.
- Candidate Key: A potential candidate for the primary key. Each candidate key can uniquely identify records, but only one becomes the primary key.
- Surrogate Key: An artificially generated key, often an auto-incremented number, used when there is no natural primary key available.