SQL · Quick Reference

Lesson 8 — JOINs: Combining Multiple Tables Cheat Sheet

SQL

In one line: Without JOINs, you'd repeat every student's name and age in every enrollment row. That's data redundancy — it wastes space and creates inconsistency when data changes.

Key Ideas

1Why JOINs Exist. Without JOINs, you'd repeat every student's name and age in every enrollment row. That's data redundancy — it wastes space and creates inconsistency when data changes.

Code Examples

-- Three related tables
CREATE TABLE courses (
  id          INT PRIMARY KEY AUTO_INCREMENT,
  course_name VARCHAR(100) NOT NULL,
  teacher     VARCHAR(100)
);

CREATE TABLE enrollments (
  id           INT PRIMARY KEY AUTO_INCREMENT,
  student_id ...

-- INNER JOIN: only rows that match in BOTH tables
SELECT s.name, c.course_name, e.enrolled_on
FROM students s
INNER JOIN enrollments e ON s.id = e.student_id
INNER JOIN courses c     ON e.course_id = c.id;

-- Shorter: JOIN = INNER JOIN
SELECT s.nam...

-- LEFT JOIN: ALL rows from left table + matching rows from right
-- Non-matching rows in right table get NULL
SELECT s.name, c.course_name
FROM students s
LEFT JOIN enrollments e ON s.id = e.student_id
LEFT JOIN courses c     ON e.course_id = c.id;
-...