The primary key in an Oracle database is a unique identifier for each row in a table. It ensures that every record in the table is distinct and can be easily retrieved.
Key Features of a Primary Key:
- Uniqueness: Each primary key value must be unique within the table.
- Non-Nullability: A primary key cannot be null (empty), guaranteeing that every row has a unique identifier.
- Immutability: Once a primary key is defined, its value cannot be changed.
Practical Examples:
- Customer Table: The primary key could be the
customer_id, ensuring each customer has a distinct identifier. - Order Table: The primary key might be the
order_id, guaranteeing that every order is uniquely identified.
Importance of a Primary Key:
- Data Integrity: The primary key maintains the integrity of data by preventing duplicate entries.
- Data Retrieval: It allows for efficient retrieval of specific records using the unique identifier.
- Relationships: Primary keys are used to establish relationships between tables in a relational database.
Defining a Primary Key:
You can define a primary key using the PRIMARY KEY constraint when creating or altering a table:
CREATE TABLE customers (
customer_id INT PRIMARY KEY,
customer_name VARCHAR(255),
...
);This code snippet creates a table named customers with customer_id as the primary key.
