Lesson 22 +10 XP

Introduction to Joins

A JOIN combines rows from two or more tables based on a related column between them.

Why join?

Data is split across tables to avoid repeating it. The Customers table has customer info, and the Orders table has order info. A join lets you see both together.

The related column


text

### Customers.CustomerID  ->  Orders.CustomerID

Orders.CustomerID is a foreign key pointing to the Customers primary key.

The main types of JOIN

Join	What it returns
INNER JOIN	matching rows in both tables
LEFT JOIN	all left rows + matching right rows
RIGHT JOIN	all right rows + matching left rows
FULL OUTER JOIN	all rows from both sides

The basic shape


sql

### SELECT columns

### FROM table1

### JOIN TYPE table2

### ON table1.column = table2.column;

The ON clause says how the tables relate.

Example


sql

### SELECT Orders.OrderID, Customers.CustomerName

### FROM Orders

### INNER JOIN Customers

### ON Orders.CustomerID = Customers.CustomerID;

TL;DR

JOIN combines rows from multiple tables.
The ON clause defines the relationship.
INNER, LEFT, RIGHT, FULL are the main types.
Joins use matching columns (usually key columns).