SQL Fundamentals · Joins · Lesson 14 of 17
LEFT JOIN
Concept
LEFT JOIN
LEFT JOIN returns every row from the left table, filling in NULLs for any columns from the right table that have no match.
By the end of this lesson
- Explain the core idea behind LEFT JOIN.
- 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.
Self-check before continuing
Without looking at the example, describe what changes when you modify one input in LEFT JOIN. 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 when you would prefer LEFT JOIN over INNER JOIN.
Need a nudge?
Ask about this lesson or your next move. The guide explains the path without taking the work away from you.
Ready for the next lesson?
Mark this lesson complete when you have finished the exercise.
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