In SQL, a key is a column or a group of columns in a table that is used to identify each row in the table. Keys are essential components of a relational database management system (RDBMS) as they help to ensure the integrity and consistency of the data stored in the database.

Uses of Keys in DBMS

There are many uses of Keys in DBMS some of them are listed below,

Uniquely identifying rows: Keys are used to identify each row in a table in a unique manner, which helps to prevent duplicate records and to ensure the accuracy and consistency of the data in the table.

Establishing relationships between tables: Keys are used to establish relationships between tables in a relational database. The primary key of one table can be used as a foreign key in another table to link the two tables together.

Enforcing data integrity: Keys are used to enforce data integrity in a table by ensuring that each row in the table is uniquely identified and that the data in the table is accurate and consistent.

Improving query performance: Keys can improve the performance of queries by allowing the database to quickly locate and retrieve the data that is needed.

Keys are an essential component of SQL that are used to ensure the integrity and consistency of the data stored in a database. They are used to identify rows, establish relationships between tables, enforce data integrity, and improve query performance.

Different Types of Keys

There are different types of keys which are used in SQL some of them are as follows

1. Primary Key
2. Foreign Key
3. Candidate Key
4. Unique Key
5. Composite Key
6. Alternate Key

Primary key: A primary key is a column or group of columns in a table that uniquely identifies each row in that table. The primary key is used to enforce the integrity of the data in the table and to prevent duplicate records.

For example, in a table of employees, the employee ID column may be designated as the primary key.

Foreign key: A foreign key is a column or group of columns in one table that refers to the primary key of another table. The foreign key is used to establish a relationship between two tables, and to ensure that the data in the two tables is consistent.

For example, in a table of orders, the customer ID column may be a foreign key that refers to the customer ID primary key in a table of customers.

Candidate key: A candidate key is a column or group of columns in a table that could potentially be used as the primary key. A table can have multiple candidate keys, but only one primary key.

For example, in a table of students, both the student ID column and the email address column could be candidate keys.

Unique key: A unique key is a column or group of columns in a table that has a unique value for each row in the table, but is not designated as the primary key. A table can have multiple unique keys.

For example, in a table of employees, the employee email address column may be designated as a unique key.

Composite key: A composite key is a combination of two or more columns in a table that together uniquely identify each row in the table.

For example, in a table of orders, a composite key might be the combination of the order ID and the order date columns.

Alternate Key: An alternate key is a candidate key that is not chosen as the primary key of a table. It is a unique identifier for each row in the table, just like the primary key, but it is not used for referential integrity or to establish relationships with other tables. Instead, it can be used for indexing or querying purposes.

For example, in table of employees, “Email” column could be designated as an alternate key, which means that it is not the primary key but can still be used to uniquely identify each row in the table.

Among all the keys above two main keys which are used most of the time are going to be Primary Key and Foreign key

Primary Key and Foreign Key Scenario based Understanding

A primary key and a foreign key are two important types of keys in a database management system (DBMS) that help to establish relationships between tables.

A primary key is a column or a set of columns in a table that uniquely identifies each row in that table. The primary key is used to enforce the integrity of the data in the table and to prevent duplicate records. For example, in a table of students, the student ID column can be designated as the primary key since each student has a unique ID number.

Here is an example of a table of students with the student ID column as the primary key:

Student ID	First Name	Last Name	Subject
001	John	Doe	Computer Science
002	Jane	Smith	English
003	Bob	Johnson	History

A foreign key is a column or a set of columns in a table that refers to the primary key of another table. The foreign key is used to establish a relationship between two tables and to ensure that the data in the two tables is consistent. For example, let’s assume that we have a table of courses with a primary key of course ID. We can create a foreign key in the table of students to establish a relationship between the two tables. In this case, the foreign key would be the course ID column in the table of students that refers to the course ID primary key in the table of courses.

Here is an example of a table of courses with the course ID column as the primary key:

Course ID	Course Name	Instructor
001	Introduction to SQL	Smith
002	English Poem – Mother	Johnson
003	World War II	Brown

Now, we can create a foreign key in the table of students to establish a relationship with the table of courses. Here is an example of the modified table of students with the course ID column as the foreign key:

Student ID	First Name	Last Name	Subject	Course ID
001	John	Doe	Computer Science	001
002	Jane	Smith	English	002
003	Bob	Johnson	History	003

In this example, the course ID column in the table of students refers to the primary key of the table of courses, which ensures that the data is consistent between the two tables.

Let’s take one more example

Let’s say you have a table called “Customers” in a database. Each record in the table represents a different customer, and you want to ensure that each customer has a unique identifier. You could create a column called “CustomerID” and designate it as the primary key for the table.

Then, when you add a new customer record to the table, you would assign a unique value to the “CustomerID” column for that record. This makes it easy to retrieve or modify individual customer records based on their unique identifier.

Also you have a second table called “Orders” that contains information about customer orders. Each order is associated with a specific customer from the “Customers” table. To establish this relationship between the two tables, you could create a column called “CustomerID” in the “Orders” table and designate it as a foreign key.

This means that the “CustomerID” column in the “Orders” table references the “CustomerID” column in the “Customers” table. When you add a new order record to the “Orders” table, you would specify the “CustomerID” value for the customer who placed the order.

Then, you can use this foreign key relationship to join the two tables together and retrieve information about specific customers and their orders.

I hope now you have understood this concepts well !! 🤔💭

Difference between Primary Key and Foreign Key

Knowing the difference between the Primary Key and Foreign Key is very important and crucial when you are learning SQL.

This are some of the points you should remember.

Conclusion

In conclusion, keys are a fundamental concept in SQL and are essential for creating relationships between tables in a database. There are different types of keys, such as primary keys, foreign keys, and composite keys, each serving a specific purpose in ensuring data integrity and maintaining data consistency.

Primary keys uniquely identify each row in a table, and foreign keys establish relationships between tables. Composite keys combine multiple columns to create a unique identifier for a row.

It’s important to carefully choose and define keys when designing a database, as they play a crucial role in ensuring the accuracy and reliability of the data. Understanding keys and their use cases will help you build efficient and robust database structures that can handle complex data relationships and queries.

I hope you have understood the key concepts of the blog.

Happy Learning !!