SQL in Primary Keys and foreign keys is essential for maintaining accuracy and structure in relational databases. These keys form the foundation of table relationships and help enforce data consistency. A Primary Key is used to uniquely identify each document within a table, while a Foreign Key connects one table to another, ensuring referential integrity. Enroll in an SQL Course in Chennai to improve your database skills and boost your career in data management. In this blog, we will discuss what is the difference between primary key and foreign key in SQL.
What is a Primary Key in SQL?
A Primary Key is a unique identifier for each record in a table. It confirms that no two rows have the same value in the column or combination of columns that make up the primary key.
Key Features of a Primary Key:
- Ensures uniqueness in the table.
- Cannot contain NULL values.
- Each table can have only one primary key.
- Used to identify records uniquely.
For example, in a table of employees, the EmployeeID column is often set as the primary key because each employee must have a unique ID.
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What is a Foreign Key in SQL?
A Foreign Key is a column or group of columns in one table that points to the Primary Key in another table. It creates a relationship between two tables, allowing one table to “reference” another.
Key Features of a Foreign Key:
- Maintains referential integrity.
- Can contain duplicate and NULL values.
- Used to link related data across tables.
- A table can have multiple foreign keys.
For example, a Department ID in the Employee table could serve as a foreign key that references the corresponding Department ID in the Departments table.
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Primary Key vs Foreign Key: Key Differences
Primary Key:
- Uniquely identifies each record in a table.
- Must contain unique values, no duplicates allowed.
- Cannot contain NULL values.
- Only one primary key is permitted per table.
- Does not reference any other table (self-contained).
- Maintains row-level uniqueness to ensure data accuracy.
Foreign Key:
- Establishes a relationship between two different tables.
- Can have duplicate values.
- Can accept NULL values.
- A table can contain multiple foreign keys.
- Guides to a primary key in another table to enforce linkage.
- Maintains referential integrity across related tables.
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Why Are These Keys Important in SQL?
Understanding and using Primary and Foreign Keys in SQL is essential for several reasons:
- They prevent data redundancy and inconsistencies.
- Enable data normalisation, making databases easier to manage.
- Help in maintaining relationships and integrity between tables.
- Support efficient data retrieval and querying.
These keys are essential when working with relational databases in real-world applications and data security strategies, such as inventory systems, student databases, e-commerce platforms, and other similar systems.
Real-Life Example for Better Clarity
Imagine a school database with two tables: Students and Courses.
- In the Students table, StudentID is the Primary Key.
- In the Courses table, there is a column called StudentID, which is a foreign key referencing the primary key in the Students table.
This relationship ensures that each course record is linked to a valid student. If someone tries to assign a course to a non-existent student ID, the database will prevent it, maintaining integrity.
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Best Practices When Using Keys in SQL
- Always define a Primary Key for every table.
- Use Foreign Keys to enforce relationships where needed.
- Avoid NULLs in foreign keys if the relationship must always exist.
- Use indexes on foreign keys for better performance.
To summarise, the primary key uniquely identifies each record in a table, ensuring data integrity. In contrast, the foreign key connects two tables, maintaining relationships and ensuring that data across tables remains consistent. Both are crucial components of relational database design and are widely used in SQL database management systems.