Last Updated on March 2, 2023 by Prepbytes
In this section we will discuss the key, primary key, and use of a primary key, foreign key, the use of a foreign key, the difference between primary key and foreign key,
What is the Key?
In database management systems, a key is a specific attribute or set of attributes used to uniquely identify each record (also known as a tuple) in a table. A key plays a crucial role in organizing data in a relational database and establishing relationships between tables. There are several types of keys in a database, each serving a different purpose.
Keys play a vital role in relational databases by uniquely identifying records, establishing relationships between tables, and improving performance.
- Primary key
- Foreign key
- Candidate key
- Composite key
Primary Key:
A primary key is a key that is used to identify a unique record in a database table. It is a fundamental component of database management, providing a way to ensure the integrity of data by ensuring that each record has a unique identifier. The primary key is used to establish relationships between tables, and it is also used to enforce data integrity by preventing the insertion of duplicate records into the table.
For example, consider a database table of employees. Each employee record in the table would have several attributes, such as name, date of birth, and job title. To identify each employee record, you could use an employee ID number as the primary key. This ID number would be unique for each employee and would be used to identify the corresponding record in the table.
The primary key is usually stored as an integer, but it can also be stored as a string, such as an employee name or a combination of several fields. It is important to choose the primary key carefully, as the key will be used throughout the database, and changing it can have a significant impact on the database structure and the relationships between tables.
In simple words, the primary key is a crucial component of database management, providing a way to identify unique records and enforce data integrity. It is important to choose the primary key carefully and to understand the impact that changes to the key can have on the database.
Syntax of Primary Key:
CREATE TABLE
(
Column1 datatype,
Column2 datatype, PRIMARY KEY (Column-Name)
.
); Foreign Key:
A foreign key is a field in a database table that is used to establish a relationship between two tables. It is a way to enforce referential integrity in a database, meaning that the data in one table must match the data in another table.
For example, we have two tables: "Customers" and "Orders". The "Customers" table contains information about each customer, including their unique customer ID. The "Orders" table contains information about each order, including the customer ID of the customer who placed the order. In this case, the customer ID in the "Orders" table is a foreign key that references the customer ID in the "Customers" table.
By using a foreign key, we ensure that every order in the "Orders" table is associated with a customer in the "Customers" table. If we try to insert an order with a customer ID that does not exist in the "Customers" table, the database will reject the insert and raise an error. This helps to prevent data inconsistencies and ensures that the data in the database is accurate and up-to-date.
In simple words, a foreign key is a field in a database table that is used to establish a relationship with another table and enforce referential integrity. By using foreign keys, we can ensure the accuracy and consistency of data in a database.
Syntax of Foreign key:
CREATE TABLE (
column1 datatype,
column2 datatype,
constraint (name of constraint)
FOREIGN KEY [column1, column2...]
REFERENCES [primary key table name] (List of primary key table column) ...);
Use of Primary Key and Foreign Key:
These are the uses of Primary key and foreign key
Primary Key Uses:
- Identifying unique records: The primary key ensures that each record in a table has a unique identifier.
- Establishing relationships: The primary key is used to link records in one table to records in another table through foreign keys, forming relationships between tables.
- Enforcing referential integrity: By linking tables through primary and foreign keys, referential integrity is maintained and ensures that data remains consistent across multiple tables.
- Improving query performance: Primary keys can be used as the basis for indexing, allowing for faster data retrieval.
- Constraining data: The primary key can be used to enforce constraints, such as ensuring that data entered into a table is unique
Foreign Key Uses:
- Establish the relationship between two tables in a database.
- Identify a unique record in one table with the related record in another table.
- Ensure referential integrity and maintain data consistency.
- Prevent data anomalies and maintain database structure.
- Facilitate data retrieval and analysis through joins.
Difference between Primary Key and Foreign Key?
These are the main difference between primary key and foreign key:
Feature
Primary key
Foreign Key
Purpose
A primary key is used to uniquely identify each record in a table
Foreign key is used to link or connect two tables together.
Uniqueness
Primary key must have unique values
Foreign key may have duplicate values.
Null values
Primary key cannot have null values
Foreign key may have null values.
Number of columns
A primary key can be one or multiple columns
A Foreign key must be one column.
Owning table
Primary key is located in the table it identifies
Foreign key is located in a different table from the one it identifies.
Relationship type
Primary key defines the parent table in a relationship
Foreign key defines the child table.
Cardinality
Primary key defines one-to-many relationship
Foreign key defines the many-to-one relationship.
Cascade options
Primary key has no cascade options
Foreign key has options for update and delete cascading.
Referential integrity
Primary key does not enforce referential integrity
Foreign key enforces referential integrity.
Indexing
Primary key is automatically indexed
Foreign key may or may not be indexed.
Conclusion
A primary key is a unique identifier for each record in a table of a relational database. It is used to ensure that each record has a unique identifier and to enforce referential integrity by linking to other tables. The primary key is typically a single column, but can also be a combination of multiple columns.
A foreign key is a field in one table that refers to the primary key of another table. It is used to establish a relationship between the two tables and enforce referential integrity. The foreign key ensures that a record in the referencing table (child table) has a corresponding record in the referenced table (parent table).
The relationship between a primary key and foreign key allows for the creation of complex data structures and the enforcement of data constraints. For example, if a record in the "Customers" table is deleted, all corresponding records in the "Orders" table will also be deleted, thus maintaining the referential integrity of the data.
The primary key and foreign key play a crucial role in the organization and management of data in relational databases, ensuring the accuracy and consistency of data across multiple tables.
FAQs Related to Primary Key and Foreign Key
These are the FAQs on the primary key and foreign key:
Question 1: Why do we use primary keys and foreign keys in databases?
Answer: We use primary keys and foreign keys to enforce data integrity and to establish relationships between tables.
Question 2: What is the difference between a primary key and foreign key?
Answer: A primary key is a unique identifier for each record in a table, while a foreign key is a column in a table that refers to the primary key of another table. The primary key ensures data integrity in its own table, while the foreign key helps to enforce referential integrity between tables.
Question 3: Can a primary key be null?
Answer: No, a primary key cannot be null because it is used to uniquely identify each record in a table.
Question 4: Can a foreign key be null?
Answer: Yes, a foreign key can be null, which means it is optional and doesn’t need to have a corresponding value in the referenced table.
Question 5: Can a table have more than one Primary Key?
Answer: No, a table can have only one Primary Key.