A foreign key is distinct from other types of keys such as super keys, candidate keys, or primary keys because its primary purpose is to establish a connection between two tables in a database. It serves as a bridge, enabling relationships and ensuring data integrity by linking the records in one table to the relevant records in another.
Foreign Key
Foreign keys are a set of constraints in DBMS that establish relationships between tables and also ensure consistency and integrity of data. A foreign key is applied to a column of one table which references the primary key of a column in another table. It act as a cross-reference between two tables. It helps to maintain data integrity which is known as "Referential Integrity Constraints".
- Note that it is essential that the referenced column in the other table must have a primary key (or at least a unique constraint). This is because the foreign key is used to link data between two tables, creating a relational structure that ensures data consistency and integrity.
- A foreign key enforces referential integrity, which ensures that the data in one table corresponds to valid entries in another table. This prevents issues such as orphaned records, where a record in the referencing table points to a non-existent record in the referenced table.
- To create a foreign key, we define the relationship between the columns during the table's structure creation. This is done by using the
REFERENCES keyword, which specifies that a particular column will reference another column in a different table. The referenced column is typically the primary key of the related table, ensuring uniqueness and enabling the foreign key relationship.
- A foreign key can refer to a column in the same table, which is called self-referencing. This is often used to show relationships within the same table, like a manager and their employees, members of a family tree, or folders inside other folders. It helps organize data in a way that shows how the rows in the table are connected to each other.
Example
Foreign Key Example- dept_id is primary key in department_info table.
- dept_id is foreign key in employee_info table.
It means that dept_id field of employee_info table can have only those value which are present in dept_id field of department_info table.
- dept_id field in department_info table is for unique identification of the records in the table.
- dept_id field in employee_info table is for knowing which employee is working on which department.
Importance of Foreign Keys
Foreign keys are essential in databases for several reasons:
- Streamline Data: Foreign keys prevent repeated data across multiple tables. They allow tables to share information without creating duplicate data, acting as a link between related tables.
- Improve Efficiency: By working with primary keys, foreign keys create a clear structure in relational databases, making it faster to sort, search, and query data.
- Maintain Data Accuracy: Foreign keys ensure data integrity by requiring that values in the foreign key column match existing values in the primary table. This ensures data stays accurate, even when changes or deletions occur in the primary table.
Syntax For Creating and Deleting Foreign Key
Let’s see the Foreign Key syntax used for creating a table.
Syntax for Creating Foreign Key:
CREATE TABLE Child_table_name (
Child_Column1 data_type,
Child_Column2 data_type, ...,
FOREIGN KEY (Child_foreign_key_column)
REFERENCES referenced_table_name (referenced_primary_key_column) );
Below syntax is used to delete the foreign key in DBMS.
Syntax for Dropping Foreign Key:
ALTER TABLE Child_table_name
DROP FOREIGN KEY Child_foreign_key_name;
Need of Foreign Keys in DBMS
Foreign keys plays a major role in database management systems (DBMS) for the following reasons:
- Data Integrity: We need foreign keys as they help us making sure that data is consistent, complete, between both the tables and overall accuracy is maintained.
- Query Optimization: Foreign keys optimizes the query execution by utilizing query plans more efficiently and improving the relationships between tables. It also helps in fast data retrieval.
- Establishing Relationships: The main requirement of foreign keys is the establishment of relationships between tables. It makes sure that data is linked across multiple tables and helps in storing and retrieving data.
- Data Security: Foreign keys helps in improving the security of data by preventing unauthorized modifications or deletions of important data in the referenced table.
- Database Maintenance: Foreign keys are required in database maintenance tasks and help to ensure integrity and consistency of data during these operations.
Difference Between Primary Key and Foreign Key
- Definition:
- A primary key uniquely identifies each record in a table.
- A foreign key links two tables by referencing the primary key of another table.
- Purpose:
- A primary key ensures each row in the table is unique.
- A foreign key establishes relationships between tables and enforces referential integrity.
- Uniqueness:
- A primary key must be unique for each record and cannot contain null values.
- A foreign key can have duplicate values and may allow null values, depending on the relationship.
- Location:
- A primary key is defined within the table it belongs to.
- A foreign key is defined in one table but references the primary key of another table.
- Example:
- Primary Key:
user_id in a Users table. - Foreign Key:
user_id in an Orders table, linking to Users.user_id.
Read more about Difference Between Primary Key and Foreign Key.
To Read more about SQL FOREIGN KEY Constraint Refer, Here.
Conclusion
In this article we have learned about foreign key in DBMS. To summarize the article, a foreign key establishes relationship between tables and ensures consistency and integrity of data . It is applied to a column of one table which references the primary key of a column in another table. It is mandatory that the other column must have a primary key as it references the data which is related in different tables and creates a relational structure. Foreign key enforces referential integrity and makes sure that data is referenced from one table to table. In order to create a Foreign key we will specify the relationship between the columns during the creation of table's structure .