SQL Joins

SQL JOIN

SQL JOIN is used to combine rows from two or more tables based on a related column between them. Joins are essential in relational databases where data is stored in multiple related tables.

Why Use JOIN?

In a relational database, information is often split into multiple tables to reduce duplication and improve structure.

• A customers table stores customer details
• An orders table stores order information

To see which customer placed which order, you need a JOIN.

Basic JOIN Syntax

SELECT columns
FROM table1
JOIN table2
  ON table1.column = table2.column;

Example: Customers and Orders

SELECT customers.name, orders.order_date
FROM customers
JOIN orders
  ON customers.id = orders.customer_id;

This query links customers to their orders using a common key.

Using Table Aliases with JOIN

SELECT c.name, o.order_date
FROM customers AS c
JOIN orders AS o
  ON c.id = o.customer_id;

Aliases make JOIN queries shorter and easier to read.

Common Types of JOIN

• INNER JOIN – returns matching rows from both tables
• LEFT JOIN – returns all rows from the left table and matched rows from the right table
• RIGHT JOIN – returns all rows from the right table and matched rows from the left table
• FULL JOIN – returns rows when there is a match in either table
• SELF JOIN – joins a table with itself

JOIN vs WHERE

Modern SQL uses JOIN with ON for linking tables. While older syntax used WHERE for joins, explicit JOIN syntax is clearer and recommended.

Foreign Keys

Joins usually connect a primary key in one table to a foreign key in another table.

Next Step

Continue with SQL INNER JOIN to understand the most commonly used join type.