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Introduction to Joins

Introduction to Joins in MySQL

In MySQL, a JOIN operation combines rows from two or more tables based on a related column between them. This allows us to query data from multiple tables as if they were a single table. MySQL provides several types of joins: INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN. In this tutorial, we will cover the basic concepts of joins and how to use them in MySQL.

What are Joins?​

In relational databases like MySQL, data is often split across multiple tables. But in many cases, you'll need to work with data from more than one table at a time. This is where joins come in. When you use a join, you combine two (or more) tables by relating them with a common field or column.

For example, consider two tables, Orders and Customers. The Orders table has a column customer_id that refers to the id field in the Customers table. We can use a join to combine these tables and get a complete view of each order and the associated customer information.

Types of Joins​

There are four basic types of joins in MySQL, and they are used in different situations depending on the result you need.

  1. Inner Join: The INNER JOIN keyword selects records that have matching values in both tables. If there is no match, the result is NULL.

  2. Left Join (or Left Outer Join): The LEFT JOIN keyword returns all records from the left table (table1), and the matched records from the right table (table2). The result is NULL from the right side, if there is no match.

  3. Right Join (or Right Outer Join): The RIGHT JOIN keyword returns all records from the right table (table2), and the matched records from the left table (table1). The result is NULL from the left side, if there is no match.

  4. Full Join (or Full Outer Join): The FULL JOIN keyword returns all records when there is a match in either left (table1) or right (table2) table records. Note that, MySQL doesn't support FULL JOIN, but you can achieve the same result using LEFT JOIN and UNION.

Basic Syntax​

Here is the basic syntax of a join operation in MySQL:

SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name = table2.column_name;

The JOIN keyword tells MySQL to combine the two tables, and the ON keyword tells it how to relate the tables.

Conclusion​

Joins are fundamental to working with relational databases like MySQL. You'll often need to use them to create meaningful queries and get the most out of your data. Understanding how to use joins effectively can greatly expand what you're able to do with MySQL. In the next tutorial, we'll dive into each type of join in more detail and provide examples of how to use them.