Today, we’ll look at the 30 most important SQL query interview questions and ideas that you need to know before your SQL coding interview.
Last summer we published the ultimate guide to the SQL Interview Questions You Must Prepare. In it, we covered all the SQL areas that are, from our experience, required by the interviewers. These, along with coding, include theoretical knowledge about SQL and databases.
Now we will concentrate only on coding skills. It’s kind of like a follow-up to the ultimate guide we talked about above, but this one is only about writing SQL queries.
This one skill will be broken down into six separate technical ideas that will be used to solve 30 business problems.
Starting with the simplest questions, we’ll add one idea to the ones that came before it. Finally, we’ll look at the most difficult questions that include most or all of the ideas at once. With five questions in each category, this means covering 30 SQL query interview questions.
Unlocking the Power of Subqueries A Comprehensive Guide to SQL Interview Success
To get information from, change, or show complicated data sets quickly and easily in SQL, subqueries are the best tool for the job. This complete guide goes into great detail about subqueries, giving you the skills and knowledge you need to do well in your upcoming SQL interview.
What are Subqueries?
Subqueries, also known as nested queries are embedded within the main SQL query, acting as dynamic and flexible instruments for retrieving data based on conditions or performing calculations on data sets. These versatile queries can return individual values or entire record lists proving invaluable in various scenarios, including
- Filtering data based on complex conditions: Subqueries enable you to filter data based on conditions that involve multiple steps of logic, making them ideal for tackling intricate data analysis tasks.
- Performing calculations on data sets: Subqueries empower you to calculate values based on data retrieved from other tables, unlocking a world of possibilities for data manipulation and analysis.
- Simplifying complex queries: Subqueries break down intricate queries into smaller, more manageable components, enhancing readability and maintainability.
Subqueries vs. Joins: Understanding the Distinction
Both subqueries and joins combine data from more than one table, but they do it in different ways and in different ways.
Subqueries:
- Nested within the main query, providing dynamic and flexible data retrieval.
- Often used when the relationship between tables is complex or not directly related.
- Can be more efficient than joins in situations where an operation needs to be performed on a subset of data before comparison with the main dataset.
Joins:
- Combine rows from multiple tables based on related columns.
- Preferred when there’s a direct relationship between tables, offering faster execution and improved readability.
Unveiling the Types of Subqueries
Subqueries come in various flavors. each tailored to specific use cases
1, Correlated Subqueries
- Depend on the outer query for their values, executing once for each row processed by the outer query.
- Useful for filtering data based on conditions that involve values from the outer query.
2. Non-Correlated Subqueries:
- Independent of the outer query, executing only once before the main query.
- Ideal for retrieving data that will be used in the main query as a condition.
3. Single-Row Subqueries:
- Return only one row from the inner SELECT statement.
- Used with operators like =, >, <, etc., for comparing values.
4. Multi-Row Subqueries:
- Return multiple rows from the inner SELECT statement.
- Used with operators like IN, ANY, or ALL, for comparing sets of values.
Subqueries in Action: Exploring Real-World Applications
Subqueries shine in various scenarios, demonstrating their versatility and power in data manipulation:
1. Finding Employees Earning More Than the Average Salary:
SELECT EmployeeNameFROM EmployeesWHERE Salary > (SELECT AVG(Salary) FROM Employees);2. Identifying Customers Who Haven’t Placed Orders:
SELECT customer_nameFROM customersWHERE NOT EXISTS (SELECT 1 FROM orders WHERE customers.customer_id = orders.customer_id);3. Determining the Second Highest Salary:
SELECT MAX(Salary)FROM EmployeeWHERE Salary NOT IN ( SELECT MAX(Salary) FROM Employee);Optimizing Subqueries for Peak Performance
To ensure your subqueries operate with optimal efficiency, consider these strategies:
- Favor joins over subqueries when possible: Joins often outperform subqueries in terms of speed and efficiency.
- Utilize the EXISTS clause instead of IN for existence checks: EXISTS stops processing once a match is found, improving performance for large datasets.
- Avoid correlated subqueries whenever possible: These subqueries execute repeatedly for each row in the outer query, potentially leading to slow performance.
- Ensure proper database table indexing: Indexes significantly speed up queries by reducing the amount of data processed.
Mastering Subqueries: A Path to SQL Interview Success
By thoroughly understanding subqueries, their types, applications, and optimization techniques, you’ll be well-equipped to tackle any subquery-related questions in your upcoming SQL interview with confidence and expertise. Remember, practice makes perfect, so don’t hesitate to experiment with different subquery scenarios to solidify your understanding and hone your skills.
Additional Resources
- Top 25 Subquery (SQL) Interview Questions and Answers: https://interviewprep.org/subquery-sql-interview-questions/
- Joins and Subqueries Interview Questions – CodeWithCurious: https://codewithcurious.com/interview/joins-and-subqueries-interview-questions/
Subqueries are an indispensable tool in the SQL developer’s arsenal, empowering you to extract, manipulate, and present data with remarkable efficiency. By mastering subqueries, you’ll not only enhance your SQL proficiency but also gain a valuable edge in your upcoming SQL interview.
SQL Query Interview Question #26: Combine the First and Last Names of Workers
When working with text data, one of the most common things you may be asked to do is join two or more table columns together. Here’s what exactly Amazon wants you to do.
Combine the first and last names of workers with a space in-between in a column full_name.Table : worker
Link to the question: https://platform.stratascratch.com/coding/9834-combine-the-first-and-last-names-of-workers
The table at your disposal is worker.
| worker_id | first_name | last_name | salary | joining_date | department |
|---|---|---|---|---|---|
| 1 | Monika | Arora | 100000 | 2014-02-20 | HR |
| 2 | Niharika | Verma | 80000 | 2014-06-11 | Admin |
| 3 | Vishal | Singhal | 300000 | 2014-02-20 | HR |
| 4 | Amitah | Singh | 500000 | 2014-02-20 | Admin |
| 5 | Vivek | Bhati | 500000 | 2014-06-11 | Admin |
You see they want you to put the employee’s first and last name in one column. Here’s how you do it:
Select the employee’s first name, then follow it up with two vertical lines ||. What follows after that will be concatenated with the first name. We put the blank space between single quotation marks because we want it to come after the first name. After that again comes the sign for concatenation, and then the employee’s last name.
This will give their names in the new column full_name.
All required columns and the first 5 rows of the solution are shown
| full_name |
|---|
| Monika Arora |
| Niharika Verma |
| Vishal Singhal |
| Amitah Singh |
| Vivek Bhati |
SQL Query Interview Question #12: Sales With Valid Promotion
Here’s a question by Meta/Facebook:
The marketing manager wants you to evaluate how well the previously ran advertising campaigns are working.
Particularly, they are interested in the promotion IDs from the online_promotions table.
Find the percentage of orders with promotion IDs from the online_promotions table applied.Tables : online_promotions, online_orders
Link to the question: https://platform.stratascratch.com/coding/2069-sales-with-valid-promotion
Here you’ll again have to JOIN two tables. This time it’s the LEFT JOIN, but it works the same way as the INNER JOIN syntax-wise. To get the percentage, you’ll have to use the COUNT() function both in the numerator and the denominator. Also, convert the result and multiply it by 100 to get the percentage with decimal points.
Here’s what your code needs to do.
All required columns and the first 5 rows of the solution are shown
| percentage |
|---|
| 75 |
Write the solution in the widget.
SQL Interview Question # 10 – What is a Sub Query?
FAQ
What is subquery in SQL interview questions?
What is subquery in SQL with examples?
What is the best practice for subqueries in SQL?
What is a subquery in a SQL interview?
In the dynamic realm of SQL databases, subqueries remain a cornerstone for complex data retrieval and manipulation. As you step into the interview spotlight, showcasing your mastery of subqueries can be a game-changer.
Should I test a subquery before integrating it into a larger query?
Test Independently: Before integrating a subquery into the larger query, test it independently to ensure it returns the expected results. Adhering to these guidelines can drastically improve the quality of your SQL queries and prepare you for job interview questions on subqueries. For more on best practices, check out SQL Best Practices.
How do I use subqueries in SQL Server?
Filtering data: Use subqueries in the WHERE clause to filter data based on specific conditions, making your queries more dynamic. Covered in subquery practice exercises 1, 2, 3, 4, 8, and 9. Nested aggregations: Employ subqueries to perform aggregations within aggregations, allowing for more complex calculations.
What interview questions are related to joins and subqueries in SQL?
Here are 50 interview questions related to Joins and Subqueries in SQL: 1. What is an SQL JOIN, and why is it important in database queries? Answer: An SQL JOIN is used to combine rows from two or more tables based on a related column between them. It’s crucial for retrieving data from multiple tables. 2.
