SQL Interview Questions for Testers With Answers and Practice Queries

Many testing interviews include a short SQL round. The interviewer slides a laptop over and asks you to find duplicate emails or the second-highest salary. These SQL interview questions for testers are the ones that come up again and again, with answers short enough to say out loud.

The goal is not to turn you into a database administrator. Testers read data, compare it with what the screen or API said, and prove or disprove a bug. If you can do that quickly, you pass most SQL rounds.

Basics first? Work through our SQL tutorial for beginners, then use this page as your revision sheet. For a wider list of testing questions, read how to write test cases too, because interviewers often mix topics.

Why interviewers ask SQL interview questions for testers

Three everyday jobs depend on SQL: checking that a form submission was saved correctly, preparing or cleaning test data, and investigating a defect in the data layer. An interviewer wants proof that you can do those without waiting for a developer.

The practice schema

All examples use two tables. Run this once in SQLite, MySQL or PostgreSQL (adjust types if your tool complains) and you can try every answer below.

CREATE TABLE employees (
  id INTEGER PRIMARY KEY,
  name VARCHAR(50) NOT NULL,
  dept VARCHAR(30),
  salary INTEGER,
  manager_id INTEGER
);

CREATE TABLE orders (
  order_id INTEGER PRIMARY KEY,
  emp_id INTEGER,
  amount DECIMAL(10,2),
  status VARCHAR(20),
  created_on DATE
);

INSERT INTO employees VALUES
 (1,'Asha','QA',70000,NULL),(2,'Ben','QA',65000,1),(3,'Chen','Dev',90000,1),
 (4,'Dina','Dev',90000,3),(5,'Eli',NULL,50000,3);
INSERT INTO orders VALUES
 (1,1,120.50,'PAID','2026-10-01'),(2,1,80.00,'PAID','2026-10-02'),
 (3,2,45.00,'FAILED','2026-10-02'),(4,3,300.00,'PAID','2026-10-03');

Basic SQL interview questions for testers

1. What is SQL and why do testers need it?

SQL (Structured Query Language) reads and changes data in relational databases. Testers use it to verify saved data, create test data and trace defects back to the database.

2. What is the difference between DELETE, TRUNCATE and DROP?

DELETE removes rows you choose with WHERE and can usually be rolled back. TRUNCATE empties the whole table quickly. DROP removes the table itself. Behaviour around rollback differs by database, so mention that you would check the one in use.

3. What is the order of a SELECT statement?

You write SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. The database logically processes FROM and WHERE first, then grouping, HAVING, SELECT and finally ORDER BY. The PostgreSQL SELECT reference documents the full syntax.

4. How do you handle NULL values?

NULL means unknown, so = NULL never matches. Use IS NULL or IS NOT NULL.

SELECT name FROM employees WHERE dept IS NULL;      -- Eli
SELECT name, COALESCE(dept, 'Unassigned') FROM employees;

5. What is the difference between WHERE and HAVING?

WHERE filters rows before grouping. HAVING filters groups after aggregation. Details and examples are in our GROUP BY and HAVING guide.

6. What is the difference between CHAR and VARCHAR?

CHAR(n) is fixed length and may pad with spaces. VARCHAR(n) stores only the characters used up to a limit. Testers care because padding can break equality checks in some databases.

7. How do you find rows that match a pattern?

SELECT name FROM employees WHERE name LIKE 'A%';    -- starts with A
SELECT name FROM employees WHERE name LIKE '_en';   -- Ben

% matches any number of characters and _ matches exactly one.

Join questions in SQL interview questions for testers

8. What are the types of joins?

INNER returns matching rows only. LEFT keeps all rows from the left table. RIGHT keeps all from the right. FULL keeps both sides. CROSS pairs every row with every row. See SQL joins explained for pictures and examples.

9. Find employees who have no orders.

SELECT e.name
FROM employees e
LEFT JOIN orders o ON o.emp_id = e.id
WHERE o.order_id IS NULL;      -- Dina, Eli

Tester angle: orphan or missing records are classic defects.

10. How do you list each employee with their manager’s name?

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

This is a self join: the same table under two aliases.

11. What is the difference between a primary key and a foreign key?

A primary key uniquely identifies a row and cannot be NULL. A foreign key points to a key in another table and enforces referential integrity. Read primary key vs foreign key for the full comparison.

Grouping and aggregate questions

Counting and grouping appear in almost every set of SQL interview questions for testers.

12. Count the employees in each department.

SELECT dept, COUNT(*) AS total
FROM employees
GROUP BY dept;

13. Which departments have more than one employee?

SELECT dept, COUNT(*) AS total
FROM employees
WHERE dept IS NOT NULL
GROUP BY dept
HAVING COUNT(*) > 1;

14. What is the difference between COUNT(*) and COUNT(column)?

COUNT(*) counts rows. COUNT(dept) counts only rows where dept is not NULL. In our data the first returns 5 and the second 4. Mixing them up is a favourite trap.

15. Find the highest salary and the second-highest salary.

SELECT MAX(salary) FROM employees;                         -- 90000
SELECT MAX(salary) FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);        -- 70000

An alternative is DISTINCT salary ORDER BY salary DESC LIMIT 1 OFFSET 1 (MySQL, PostgreSQL, SQLite) or DENSE_RANK() in databases that support window functions.

Subqueries, duplicates and sets

16. How do you find duplicate values?

SELECT salary, COUNT(*) AS times
FROM employees
GROUP BY salary
HAVING COUNT(*) > 1;     -- 90000 appears twice

On a real project run this on an email or order-number column that should be unique.

17. What is a subquery?

A query inside another query. Example: employees earning above the average.

SELECT name FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

18. What is the difference between UNION and UNION ALL?

UNION removes duplicate rows from the combined result. UNION ALL keeps them and is faster because it skips the duplicate check.

19. What is the difference between IN and EXISTS?

Both test membership. IN compares against a list of values. EXISTS asks whether a correlated subquery returns any row. For large tables EXISTS often reads better, though modern optimizers handle both well. Be careful with NOT IN when the list contains NULL: it can return no rows at all.

Data validation scenarios

Many SQL interview questions for testers are really scenarios. Here are three that sound like daily work.

20. A user placed an order but it does not show on the screen. How do you check the database?

SELECT * FROM orders
WHERE emp_id = 2
ORDER BY created_on DESC;

Say what you would compare: the row’s existence, its status and amount, and the timestamp, against the UI and the API response. Our API testing with Postman guide covers the API half.

21. How do you check that migrated data matches the source?

-- row counts
SELECT COUNT(*) FROM old_orders;
SELECT COUNT(*) FROM new_orders;

-- rows present in old but missing in new
SELECT o.order_id FROM old_orders o
LEFT JOIN new_orders n ON n.order_id = o.order_id
WHERE n.order_id IS NULL;

Also compare totals such as SUM(amount) and sample a few rows field by field.

22. How do you make sure your test did not leave junk data?

Create records with a recognisable marker, for example an email ending in @test.example, and delete them afterwards with a narrow WHERE. Always run the matching SELECT first, and never run an update or delete without a WHERE clause on shared data.

Advanced SQL interview questions for testers

Senior roles add a few extras. Be ready to explain these in one or two sentences:

• Index: a structure that speeds up lookups at the cost of slower writes and extra storage.
• Transaction and ACID: a group of statements that succeed or fail together; atomicity, consistency, isolation, durability.
• View: a saved query that behaves like a table.
• Window functions: ROW_NUMBER(), RANK() and DENSE_RANK() number or rank rows without collapsing them.
• Execution plan: the database’s description of how it will run a query, useful when a screen is slow. In PostgreSQL and MySQL use EXPLAIN.

The SQLite language reference is a handy free place to check syntax for any of these.

How to answer SQL interview questions for testers well

• Clarify first. Ask which database and whether names can be duplicated.
• Write simple SQL. A readable query beats a clever one.
• Say how you would verify it. Mention running it on a small sample and comparing with the UI.
• Admit what you do not know. Say which function you would look up rather than guessing.
• Practise by typing. Reading answers is not the same as writing them under pressure.

Final thoughts

You do not need hundreds of questions. Master the twenty-two above on one small schema, practise explaining what each query would prove, and these SQL interview questions for testers will feel routine. Then try your own variations: another table, another join, another trap.

Frequently asked questions

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