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Lesson 22 +10 XP

Introduction to Joins

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

Customers.CustomerID  ->  Orders.CustomerID

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

The main types of JOIN

JoinWhat it returns
INNER JOINmatching rows in both tables
LEFT JOINall left rows + matching right rows
RIGHT JOINall right rows + matching left rows
FULL OUTER JOINall rows from both sides

The basic shape

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

The ON clause says how the tables relate.

Example

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).