SQL Joins Interview Questions 2026: 26 Q&A With Queries

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~16 min

Joins are where SQL interviews get real, because they reveal whether you understand set logic and NULL semantics. Candidates report that inner versus outer joins, self joins, and the WHERE-versus-ON trap are guaranteed for data analyst and backend roles. This guide covers 26 join questions with full queries and predict-the-result traps. Behaviour reflects ANSI SQL and the major engines; confirm any engine-specific detail on the official vendor docs.

Pair this with SQL Interview Questions 2026 and SQL Query Output Questions 2026.

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Sample Tables Used Throughout

-- employees
| emp_id | name    | dept_id |
|--------|---------|---------|
| 1      | Alice   | 10      |
| 2      | Bob     | 20      |
| 3      | Charlie | 10      |
| 4      | Diana   | NULL    |

-- departments
| dept_id | dept_name   |
|---------|-------------|
| 10      | Engineering |
| 20      | Marketing   |
| 30      | Finance     |

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Table of Contents

1. Join Types (Q1 to Q10)
2. NULLs and Filters (Q11 to Q17)
3. Self and Advanced Joins (Q18 to Q23)
4. Performance (Q24 to Q26)
5. Predict the Result
6. Join Cheat Sheet
7. Frequently Asked Questions

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Join Types

Q1. What is a JOIN in SQL? Easy

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Q2. What is an INNER JOIN? Easy

SELECT e.name, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id;
-- Alice/Engineering, Bob/Marketing, Charlie/Engineering

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Q3. What is a LEFT JOIN? Easy

SELECT e.name, d.dept_name
FROM employees e
LEFT JOIN departments d ON e.dept_id = d.dept_id;
-- includes Diana / NULL

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Q4. What is a RIGHT JOIN? Medium

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Q5. What is a FULL OUTER JOIN? Medium

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Q6. What is a CROSS JOIN? Medium

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Q7. What is the difference between INNER JOIN and LEFT JOIN? Easy

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Q8. What is a NATURAL JOIN? Medium

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Q9. Can you join more than two tables? Easy

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Q10. What is the difference between JOIN and UNION? Medium

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NULLs and Filters

Q11. How do NULLs behave in a join condition? Hard

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Q12. What is the WHERE vs ON difference in outer joins? Hard

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Q13. How do you find employees with no department? Medium

SELECT e.name
FROM employees e
LEFT JOIN departments d ON e.dept_id = d.dept_id
WHERE d.dept_id IS NULL;   -- Diana

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Q14. How do you find departments with no employees? Medium

SELECT d.dept_name
FROM departments d
LEFT JOIN employees e ON d.dept_id = e.dept_id
WHERE e.emp_id IS NULL;   -- Finance

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Q15. What does COALESCE do in a join result? Medium

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Q16. Why might a join produce duplicate rows? Hard

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Q17. What is a semi join and an anti join? Hard

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Self and Advanced Joins

Q18. What is a self join? Medium

SELECT e.name AS emp, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.emp_id;

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Q19. How do you find pairs of employees in the same department? Hard

SELECT a.name, b.name
FROM employees a
JOIN employees b ON a.dept_id = b.dept_id AND a.emp_id < b.emp_id;

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Q20. What is a join with an aggregate? Medium

SELECT d.dept_name, COUNT(e.emp_id) AS headcount
FROM departments d
LEFT JOIN employees e ON d.dept_id = e.dept_id
GROUP BY d.dept_name;

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Q21. What is the difference between joining in ON vs a subquery? Hard

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Q22. Can you join on multiple columns? Medium

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Q23. What is an equi join vs a non-equi join? Medium

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Performance

Q24. How do indexes affect joins? Medium

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Q25. What are the physical join algorithms? Hard

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Q26. Why can a LEFT JOIN be slower than expected? Hard

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Predict the Result

Snippet 1

SELECT COUNT(*) FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id;

Result: 3 Why: Only Alice, Bob, and Charlie have matching departments; Diana (NULL dept_id) is excluded by the inner join.

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Snippet 2

SELECT COUNT(*) FROM employees e
LEFT JOIN departments d ON e.dept_id = d.dept_id;

Result: 4 Why: LEFT JOIN keeps all employees including Diana, who gets NULL department columns.

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Snippet 3

SELECT COUNT(*) FROM employees CROSS JOIN departments;

Result: 12 Why: Cartesian product of 4 employees and 3 departments is 4 times 3 equals 12 rows.

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Join Cheat Sheet

Join	Returns	Unmatched rows
INNER	matches only	dropped both sides
LEFT	all left + matches	NULL on right
RIGHT	all right + matches	NULL on left
FULL OUTER	all rows	NULL where unmatched
CROSS	every combination	n/a
SELF	table to itself	depends on join type

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Frequently Asked Questions

What join question is asked most in 2026?

Candidates report the difference between INNER JOIN and LEFT JOIN, how NULLs behave in outer joins, and writing a self join are the three most repeated SQL join questions across analyst and backend interviews.

Do interviewers ask for the result of a join on sample tables?

Frequently. Predicting the exact rows of an inner versus left join, including unmatched NULL rows, is a standard whiteboard question, so practise tracing joins by hand.

What is the difference between WHERE and ON in a join?

ON filters during the join and preserves outer-join NULL rows, while a WHERE clause on the right table after a LEFT JOIN can silently turn it into an inner join by removing the NULL rows.

How do I find rows with no match in another table?

Use the anti-join pattern: LEFT JOIN the two tables and keep rows where the right side key IS NULL. The same idea reversed finds departments with no employees.

Why does my join return duplicate rows?

Because the join key is not unique on one side, so each left row matches several right rows and multiplies. Join on a unique key, aggregate, or apply DISTINCT to control duplication.

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Related Articles

• SQL Interview Questions 2026, the master 50-question set
• SQL Query Output Questions 2026, predict-the-result drills
• SQL Indexing Interview Questions 2026, performance tuning
• DBMS Interview Questions 2026, the theory side

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Confirm any engine-specific behaviour on the official vendor documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.