SQL Fundamentals · Joins · Lesson 12 of 17
Why Joins?
Concept
Why Joins?
Relational data is split across multiple tables to avoid repetition. A JOIN combines rows from two tables based on a shared column.
By the end of this lesson
- Explain the core idea behind Why Joins?.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
How to study this page
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
ExampleRunnable
-- students(id, name)
-- enrollments(student_id, course_id)
-- Join them on students.id = enrollments.student_idSelf-check before continuing
Without looking at the example, describe what changes when you modify one input in Why Joins?. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Explain, as a comment, why storing enrollments in a separate table avoids duplicating student data.
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Console