Joining Tables

Imagine we have to generate a report of our online store showing customer names alongside the products they ordered. Because customer details are stored in the Customers table and product details in the Products table, we need a way to combine data from multiple tables. This is where joins in SQL help us. They combine information from multiple tables into a single result.

Let's learn to write queries that merge data from several tables, making our data analysis far more powerful and flexible. We'll focus to:

• Understand what joins are and why they are important in SQL.

• Learn about different types of SQL joins: INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

• Learn to perform basic and multiple-table joins using the course database.

Understanding table joins

Joins allow us to retrieve related information spread across different tables by linking them through a common column, typically a primary key and a foreign key relationship. The primary key uniquely identifies each record in a table, ensuring data integrity, while the foreign key establishes a connection between related tables by referencing the primary key of another table. Without joins, we would have to store all data in a single table, which would be inefficient and against the principles of relational databases. By using joins, we keep data organized and only combine it when required, ensuring consistency and reducing redundancy. The primary-foreign key relationship is crucial in enforcing data integrity and optimizing query performance, making it easier to maintain and update the database efficiently.

Without joins, we would have to manually query each table and piece together the data, which is time-consuming and error-prone. Joins streamline this process and ensure accurate relationships are maintained.

The key to a join is a shared field. For instance, in the OnlineStore database, the Orders table stores CustomerID. The Customers table also contains CustomerID. Using that common CustomerID, we can connect details about orders with information about the customers who placed them.

Types of SQL joins

There are ...