Cracking database interview questions isn’t only about memorizing syntax or knowing the difference between INNER JOIN and LEFT JOIN. What really sets strong candidates apart is their ability to avoid common mistakes that signal inexperience to interviewers.
Even if you know SQL well, slipping on fundamentals can cost you the role. Let’s look at the top mistakes candidates make with SQL and database design—and how to avoid them.
It’s tempting to pull all columns when writing queries. But SELECT * is often seen as lazy and inefficient.
Why it’s bad:
Fetches unnecessary data.
Slows down queries on large tables.
Makes queries harder to maintain.
Better approach:
SELECT employee_id, name, salary
FROM employees
WHERE status = 'Active';
Interview insight: Specificity shows control. In database interview questions, interviewers want to see that you query with intention.
Joining tables incorrectly is a classic interview trap.
Missing the ON clause leads to a Cartesian product.
Using the wrong type of join can eliminate or duplicate data.
Wrong:
SELECT e.name, d.department_name
FROM employees e, departments d;
Correct:
SELECT e.name, d.department_name
FROM employees e
JOIN departments d
ON e.department_id = d.department_id;
Tip: Always double-check the join condition.
NULLs behave differently in SQL, but candidates often forget.
Wrong:
SELECT *
FROM employees
WHERE manager_id = NULL;
Correct:
SELECT *
FROM employees
WHERE manager_id IS NULL;
Why it matters: Many database interview questions are designed to test if you understand NULL handling. Don’t fall into this trap.
Some candidates write deeply nested subqueries when a cleaner approach would work.
Example with CTE (better readability):
WITH avg_salary_per_dept AS (
SELECT department_id, AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id
)
SELECT e.employee_name, e.salary
FROM employees e
JOIN avg_salary_per_dept d
ON e.department_id = d.department_id
WHERE e.salary > d.avg_salary;
Interview tip: Clear and readable queries are always preferred over unnecessarily complex ones.
Candidates often focus on correctness but ignore performance.
Without index:
SELECT * FROM orders WHERE customer_id = 5001;
With index:
CREATE INDEX idx_customer_id ON orders(customer_id);
SELECT order_id, order_date
FROM orders
WHERE customer_id = 5001;
Why it matters: Even if interviewers don’t expect you to create indexes on the spot, mentioning them shows awareness of efficiency.
Poor schema design is a red flag. Common issues include:
Storing multiple values in one column.
Not normalizing data.
Using inconsistent naming.
Better practice: Break many-to-many relationships into separate tables and maintain consistency in naming.
Interview tip: In design-based database interview questions, always explain your reasoning.
Many candidates write queries that work for the “happy path” but fail for corner cases.
For example:
How does your query behave with empty tables?
What if the data contains duplicates?
What happens if there are NULLs in key columns?
Pro tip: Always test your queries mentally against small datasets before presenting them.
When preparing for database interview questions, remember: the interviewer isn’t only checking if your query runs. They want to see:
Do you write efficient, maintainable SQL?
Can you design schemas thoughtfully?
Are you mindful of performance and edge cases?
Avoiding these mistakes will instantly make your answers stronger and your confidence higher. Instead of just knowing SQL, you’ll show that you understand how to apply it like a professional.