SQL Interview Questions and Tasks
Today, SQL is one of the most popular languages, actively used in many professions. Analysts in various fields, data engineers, testers, DevOps engineers, and others all face SQL-related interview questions. It’s important not only to understand the theoretical aspects of SQL but also to be able to apply them in practice. But what exactly should candidates expect at different stages of the hiring process?
In this article, we’ll cover typical interview questions, sample SQL tasks, and the specifics of the test, technical, and final interviews.
What stages does a candidate go through?
The hiring process can vary greatly depending on the position and company, but it typically consists of several stages, each aimed at assessing a candidate’s various skills: from a thorough understanding of theoretical foundations to in-depth knowledge and the ability to apply them in practice. Let’s look at the most popular ones.
1. Telephone or online interview (Screening)
Often, the first stage of an interview is a brief phone or online conversation with a recruiter or HR representative. The main goal of this stage is to determine whether your experience and skills match the job requirements.
SQL Questions at this stage:
SQL is typically only briefly touched upon at this stage. You may be asked about your experience working with databases, the types of queries you’ve used, and how often you’ve used SQL in your work. Examples of questions:
- What volume of data do you typically work with?
- Which DBMS do you prefer and why?
- How familiar are you with indexes and their uses?
Sometimes, even at this stage, you may be asked more practical questions, but usually with simple answers that an HR specialist can verify. For example:
- We’re given two tables: the first has only 5 rows, the second has 10. We’re joining them using a LEFT JOIN. What are the minimum and maximum number of rows that can appear in the resulting table?
Answer:
The table can contain from 5 to 50 rows.
2. Technical interview
This stage is typically conducted with the technical lead or another specialist, who dives deeper into your technical background. This is where the main assessment of your SQL knowledge begins and can include both theoretical questions and query writing challenges.
SQL Questions at this stage:
- Explain the difference between INNER JOIN, LEFT JOIN, and FULL OUTER JOIN.
Example answer:
- INNER JOIN : Returns only those records that have matching values in both tables involved in the join. If a record is missing from either table, it will not be included in the result.
- LEFT JOIN : Returns all records from the left table (the one specified first in the query), even if there are no matching records in the right table. If there is no matching record in the right table, NULL values will be returned instead.
- FULL OUTER JOIN : Returns all records from both tables, regardless of whether there are matching records in the other table. If there are no matching records, NULL values are returned for the missing data.
- How does GROUP BY work, and what aggregation functions can be used with it?
Example answer:
GROUP BY : This operator groups rows with identical values in the specified columns into one or more rows. This allows you to apply aggregate functions such as COUNT, SUM, AVG, MAX, MIN, and others to each group of data.
- What is an index in a database, and how does it affect query performance?
Example answer:
- An index is a data structure in a database used to quickly search and access data in a table. Indexes are created on one or more table columns and help speed up queries by reducing the number of rows that need to be scanned.
- Positive impact on performance: Indexes significantly speed up search (SELECT), sort (ORDER BY), and join (JOIN) operations because they allow the database to quickly find the rows it needs.
- Negative impact on performance: However, indexes can slow down INSERT, UPDATE, and DELETE operations because every time an indexed field changes, the index itself must be updated.
- How does COALESCE work, and where can it be used?
Example answer:
- COALESCE takes one or more arguments and returns the first one that is not NULL. If all arguments are NULL, NULL is returned.
- Handling NULL values: COALESCE is often used to supply a default value if a field contains NULL.
- Simplifying complex expressions: COALESCE can be used in complex environments to simplify the handling of multiple possible NULL values in a single expression.
SQL Tasks at this stage:
Depending on the position level, tasks at this stage may involve writing SQL queries of varying levels. You may be asked to solve a problem in real time and explain your logic and approach to the solution. Let’s look at some examples of interview tasks:
Easy task (taken from a real test at Alfa-Bank):
You are given the following table structure:
It is necessary to display the number of people who purchased a product with id = 5 after October 10, 2021 (inclusive).
Answer:
SELECT COUNT(DISTINCT user_id)
FROM purchases
WHERE sku_id = 5
AND created_at >= '2021-10-10'- Average task:You are given the following table structure:
Select the product ID with the highest number of orders, broken down by year.
Answer:
WITH sales AS
(
SELECT TO_CHAR(ord_datetime, 'YYYY') as year, a.an_id, COUNT(DISTINCT o.ord_id) as cnt
FROM Orders o
JOIN Analysis a
ON o.ord_an = a.an_id
GROUP BY TO_CHAR(ord_datetime, 'YYYY'), a.an_id
),
ranks AS
(
SELECT *, RANK() OVER(PARTITION BY year ORDER BY cnt DESC) as rnk
FROM sales
)
SELECT year, an_id, cnt
FROM ranks
WHERE rnk = 1
ORDER BY year, an_idComplex task (taken from a real test at Alfa-Bank):
You are given the following table structure:
It is necessary to calculate the average cost of the 5th purchase, broken down by city.
Answer:
WITH rn AS (
SELECT *, ROW_NUMBER() OVER(PARTITION BY c.town, c.id_customer ORDER BY p.created_at, p.id) AS rn
FROM purchases p
JOIN skus s
ON p.sku_id = s.id
JOIN customer c
ON p.user_id = c.id_customer
),
filtered_rn AS (
SELECT *
FROM rn
WHERE rn = 5
)
SELECT town, AVG(price) AS avg_price_5th_purchase
FROM filtered_rn
GROUP BY town
ORDER BY avg_price_5th_purchase DESC3. Test task
This isn’t a mandatory selection step, but after completing the technical interview (and sometimes after the phone interview), candidates may be asked to complete a test task. Typically, this task is more comprehensive than the tasks in the technical interview and may involve data analysis, conclusions, and creating reports based on SQL queries.
SQL Tasks at this stage:
The test assignment often includes more complex tasks requiring multi-level query writing, code optimization, and data analysis. Example assignment:
You are working with tables that represent a standard e-commerce database structure:
- orders (order information):
- order_id: unique order identifier.
- customer_id: customer ID
- order_date: order date
- status: order status
- order_items (information about products in orders):
- order_item_id: unique identifier of the item in the order
- order_id: order ID
- product_id: product identifier
- quantity: quantity of goods
- price: the price per unit of goods
Exercise:
- Write a query that returns all orders containing more than 5 units of the same product type.
- Write a query that returns the total number of products sold and the total revenue for each product for the last month.
- Write a query that returns the percentage of canceled orders compared to the total number of orders over the last three months.
- Write a query to detect potential data problems (e.g. orders without products or products without orders).
4. Final interview
During the final interview stage, candidates typically meet with managers or senior management. While these meetings may not address technical issues, they may discuss previous test results and general problem-solving approaches.
SQL Questions at this stage:
This can include higher-level discussions about how SQL is used within the company, how to optimize database operations, and how you can improve your company’s analytics processes. For example:
- How would you organize the ETL process for our project?
- How do you deal with SQL query performance issues in large databases?
- How do you ensure data integrity when working with transactions?
SQL Tasks at this stage:
Most often, the tasks at this stage will be similar to those you solved during the technical interview.
Conclusion
Now you know exactly which SQL questions and tasks to prepare for your interview. Remember, it’s important not only to understand the theory but also to be able to apply your knowledge in practice, demonstrating an understanding of business objectives and an ability to optimize data processing. You can practice solving SQL problems on our platform – we have SQL and Python tasks of varying levels, including examples from real interviews at top companies!
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