Essential SQL Queries You Should Know Before an Interview

“SQL is dead. Why learn an ancient language when you have Python and R?”

Hold on! Let’s set the record straight. SQL is far from dead. In fact, it’s very much alive, kicking, and ruling the data world. Ask any data professional, and they’ll tell you that SQL is still the backbone of data manipulation, analysis, and querying in almost every organization. If you’re prepping for a data interview and ignoring SQL, you’re missing out big time.

Despite all the fancy new tools, SQL remains the most common language you’ll be tested on. In this blog, we’ll cover the essential SQL queries that will help you ace your next interview!

1. SELECT: Your First Step into SQL

When it comes to SQL, nothing starts without a SELECT statement. It’s the bread and butter of querying databases. Whether you’re working on a massive dataset or just pulling simple records, SELECT is the foundation. Here’s a basic example:

SELECT * FROM employees;

This query fetches all data from the employees table. It might seem basic, but trust me—interviewers love to test variations of the SELECT query, so master this inside and out.

 

Pro Tip: Practice using SELECT with specific columns instead of * to improve efficiency.

 

2. WHERE: Filtering the Data

Now, imagine you’re dealing with millions of rows—clearly, you don’t want all of them. This is where the WHERE clause comes in, allowing you to filter out irrelevant data.

SELECT * FROM employees WHERE department = 'HR';

This query fetches only the rows where the department is HR. Interviewers may ask you to use complex conditions, so make sure you’re comfortable with logical operators like AND, OR, and NOT.

 

Common Interview Question: Fetch records where the salary is greater than $50,000 AND the department is ‘Finance.’ Can you write the query?

 

3. JOIN: Combining Data Across Tables

Almost every SQL interview will test your knowledge of JOIN. Why? Because real-world data is almost always spread across multiple tables, and understanding how to combine them is critical.

SELECT employees.name, departments.department_name
FROM employees
JOIN departments
ON employees.department_id = departments.department_id;

This query retrieves employee names along with their respective department names by combining two tables. Interviewers might ask for different types of joins like LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN, so be sure you know their differences.

 

Controversial Tip: The question of which JOIN is best for performance will come up, but there’s no one-size-fits-all answer. It depends on the use case!

 

4. GROUP BY: Aggregating Data Like a Pro

When you want to group records and apply aggregate functions like COUNT, SUM, AVG, etc., the GROUP BY clause is your best friend. It’s a must-know for any SQL interview.

 

SELECT department, COUNT(*)
FROM employees
GROUP BY department;

 

This query gives you a count of employees in each department. Interviewers often combine GROUP BY with HAVING to challenge you further.

 

Common Question: Can you find departments with more than 10 employees? You’ll need to use HAVING here to filter grouped data!