45+ SQL Interview Questions & Answers for Data Analyst Role

SQL (Structured Query Language) plays a vital role in data analytics, serving as a fundamental tool for extracting, transforming, and analyzing large datasets. Its importance lies in its ability to efficiently query and manipulate data stored in relational databases, enabling data analysts to uncover insights and patterns that inform business decisions. 

SQL syntax allows users to specify what data they want to retrieve, rather than how to retrieve it, making it an ideal language for complex data analysis tasks. With SQL, analysts can perform various operations such as filtering, sorting, grouping, and aggregating data, as well as joining tables to combine data from multiple sources. This enables them to answer complex questions, identify trends, and create data visualizations that drive business intelligence. 

Moreover, SQL’s ability to handle large datasets and scale with big data environments makes it an essential skill for data analysts and scientists working with massive datasets. Overall, SQL’s importance in data analytics lies in its ability to facilitate efficient, flexible, and scalable data manipulation, ultimately driving data-driven decision-making and business success.

This guide provides a comprehensive collection of SQL interview questions designed for Data Analysts. It covers a wide range of topics, from fundamental concepts to more advanced techniques, ensuring a thorough understanding of SQL. The questions are organized into two main categories: General SQL Questions and Technical SQL Questions. 

The technical section further explores specialized areas like SQL Functions, Advanced Commands, Database Design, and Advanced Queries, including nested and correlated subqueries. This structure helps to break down complex topics, making it easier for candidates to prepare effectively for SQL interviews.

Here are the most asked SQL Interview questions for the Data Analyst role from basic to advanced level:

Basic SQL Interview Questions

1. What is SQL? 

SQL (Structured Query Language) is a domain-specific language used to manage and manipulate relational databases. It allows users to query, insert, update, and delete data, as well as manage database structures, including tables and relationships.

2. What is an index in SQL? 

An index is a data structure that improves the speed of data retrieval operations on a table. It allows the database to find rows more efficiently, but indexes also require additional storage and can slow down data modification operations.

3. What is a primary key? 

A primary key is a distinct value that uniquely identifies each record within a table. It ensures that no two rows in a table have the same value for the primary key column(s) and helps maintain the integrity of the data.

4. What is a foreign key?

A foreign key is a column or a group of columns in one table that links to the primary key of another table. It establishes a relationship between two tables, ensuring data consistency and integrity. In other words, a foreign key links a row in one table (the child table) to a row in another table (the parent table) based on a matching value.

5. What is a JOIN in SQL? 

A JOIN operation is used to combine rows from two or more tables based on a related column. There are several types of JOINs, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN, each designed to combine data in different ways.

6. What is a Composite Key?

A composite key is a primary key composed of two or more columns. It is utilized when a single column cannot uniquely distinguish records in a table. Each combination of values in the composite key must be unique.

7. What is a transaction in SQL? 

A transaction in SQL is a single unit of work that consists of one or more SQL statements that are executed as a single, all-or-nothing unit. It contains several operations which are carried out in an atomic and consistent way, and are essential for preserving the integrity of the database.

8. What is a subquery in SQL? 

A subquery is a query embedded within another query. It is often used in SELECT, INSERT, UPDATE, or DELETE statements to retrieve data that is then used in the main query to perform operations like filtering or comparison.

Example

SELECT product_name FROM products 
WHERE price > (SELECT AVG(price) FROM products);

9. What is normalization in SQL? 

Normalization is the technique of structuring data in a relational database to minimize redundancy and dependency. It divides large tables into smaller, manageable ones, ensuring data integrity and efficient storage.

10. What is a SELECT statement in SQL? 

A SELECT statement is used to fetch data from one or more tables. It allows specifying which columns to retrieve, applying conditions (WHERE clause), sorting the results (ORDER BY), and grouping them (GROUP BY) based on certain criteria.

Technical SQL Interview Questions

11. What are aggregate functions in SQL? 

In SQL, aggregate functions perform calculations on a collection of values and provide a single summary result These functions are often used with the GROUP BY clause to summarize data and extract insights from large datasets. Common aggregate functions include COUNT(), SUM(), AVG(), MAX(), MIN() and GROUPING SETS(). These functions are commonly used with the GROUP BY clause to group and summarize data.

12. What is the HAVING clause in SQL? 

The HAVING clause is used to filter groups of rows that are generated by the GROUP BY clause. Unlike the WHERE clause, which filters individual rows, HAVING filters after the aggregation process.

SELECT product_type, COUNT(*) FROM inventory 
GROUP BY product_type HAVING COUNT(*) > 5;

13. What is a Cursor in SQL?

A cursor is a database object used to retrieve, manipulate, and navigate through rows of a result set one at a time. Cursors are useful for operations that need row-by-row processing in SQL, though they can be slower than set-based operations.

14. What is a Recursive Query?

A recursive query is a query that calls itself. In SQL, recursive queries are typically written using a Common Table Expression (CTE). They are useful for hierarchical data, like organizational charts or folder structures.

15.  What is Denormalization?

Denormalization is the process of introducing redundancy into a database by merging tables or adding redundant columns. It is often used to improve performance by reducing the need for complex joins in read-heavy databases.

16. What is the purpose of the GROUP BY clause? 

The GROUP BY clause groups rows that have the same values into summary rows, often used with aggregate functions like COUNT, SUM, or AVG. It is essential for analyzing data in chunks based on a specific column.

SELECT product_type, COUNT(*) FROM inventory GROUP BY product_type;

17. What differentiates a function from a stored procedure in SQL?

A function returns a single value and can be used in a SELECT statement, while a stored procedure can perform multiple operations and doesn't necessarily return a value. Stored procedures can also accept input/output parameters.

18. What is the difference between DELETE and TRUNCATE in SQL? 

The comparison of DELETE and TRUNCATE in SQL is:

19. How do you alter the structure of a table in SQL?

Use the ALTER TABLE statement to modify a table, such as adding or removing columns.

ALTER TABLE products ADD COLUMN stock_quantity INT;

20. What is a unique constraint in SQL? 

In SQL, a unique constraint is a database constraint that ensures a column or set of columns (composite unique key) in a table contains unique values. In other words, no two records in the table can have the same value(s) for the specified column(s).

A unique constraint enforces the following rules:

• Uniqueness: Each value in the specified column(s) must be distinct.
• Nullability: Some columns within the unique constraint can contain null values as long as the combination of values is unique.

21. What is the purpose of the EXPLAIN command in SQL? 

The EXPLAIN command is used to show the execution plan of a query. It provides details about how the database engine processes the query, helping to optimize performance by identifying inefficiencies like full table scans.

22. What is a view in SQL? 

A view is a virtual table generated by a query that fetches data from one or more tables. It simplifies complex queries, provides data abstraction, and can be used to restrict access to specific data.

23. What distinguishes CHAR from VARCHAR?

CHAR is a fixed-length data type, while VARCHAR is variable-length.CHAR adds spaces to the string to reach the specified length, while VARCHAR only allocates space for the actual length of the string.

24. What are window functions in SQL? 

Window functions perform calculations over a set of rows in a table that are related to the current row. They are commonly used for operations such as ranking, cumulative totals, and moving averages. Some common window functions are ROW_NUMBER(), RANK(), and SUM() OVER().

25. What is the difference between a CROSS JOIN and an INNER JOIN? 

A CROSS JOIN returns the Cartesian product of two tables, meaning every row of the first table is paired with every row of the second table. An INNER JOIN returns only matching rows based on a specified condition.

Example

CROSS JOIN
SELECT employees.name, departments.name 
FROM employees CROSS JOIN departments;

26. What is a self-join in SQL? 

A self-join occurs when a table is joined with itself. It is typically used to compare rows within the same table, often requiring an alias to differentiate between the two instances of the same table.

27. Explain the ACID properties in SQL transactions. 

ACID stands for Atomicity, Consistency, Isolation, and Durability:

• Atomicity: A transaction is fully completed or not at all.
• Consistency: Transactions ensure that the database transitions from one valid state to another.
• Isolation: Transactions execute independently without interference.
• Durability: Once committed, changes are permanent.

28. What is a recursive CTE in SQL? 

A recursive Common Table Expression (CTE) is used to perform recursive queries, such as traversing hierarchical data. It references itself to build a result set by repeatedly executing a query until the base case is reached.

29. What is the difference between a clustered and a non-clustered index? 

A clustered index sorts the data rows of the table based on the indexed column, and there can only be one clustered index per table. A non-clustered index creates a separate structure to point to the data rows, allowing multiple non-clustered indexes.

30. What is a database partition in SQL?

Database partitioning involves dividing a large database into smaller, more manageable segments known as partitions. This can improve query performance and manageability by distributing data across different storage locations based on certain criteria like range or hash.

31. What is an alias in SQL? 

An alias is a temporary name given to a table or a column in a query for better readability. It is defined using the AS keyword. Aliases are often used in SELECT statements or JOINs to simplify complex queries.

32. What is the difference between a UNION and a JOIN? 

A UNION combines the result sets of two or more SELECT queries into a single result, removing duplicates. A JOIN, on the other hand, combines rows from two or more tables based on a related column.

33. What is the difference between a stored procedure and a trigger? 

A stored procedure is a precompiled SQL statement that can be executed manually or automatically. A trigger is an automatic action that is executed in response to an event on a table (INSERT, UPDATE, DELETE).

34. How do you retrieve all columns from a table named employees?

SELECT * FROM employees;

35. Write an SQL query to select distinct values from the region column.

SELECT DISTINCT region FROM sales;

36. Explain how the CASE statement works in SQL.

The CASE statement allows conditional logic in SQL queries.

SELECT product_name, 
       CASE 
           WHEN price > 100 THEN 'Expensive' 
           ELSE 'Affordable' 
       END AS price_category
FROM products;

37. How can you handle NULL values in SQL?

Use IS NULL, IS NOT NULL, or COALESCE() to handle NULL values.

SELECT COALESCE(discount, 0) FROM sales;

38. How do you join two tables?

Use the JOIN clause to combine rows from two tables based on a related column.

SELECT orders.order_id, customers.name 
FROM orders 
JOIN customers ON orders.customer_id = customers.id;

39. How do you identify duplicate records in a table?

Use GROUP BY and HAVING with COUNT(*) to identify duplicates.

SELECT product_name, COUNT(*) 
FROM sales 
GROUP BY product_name 
HAVING COUNT(*) > 1;

40. What does the EXISTS clause do?

The EXISTS clause is a logical operator used in SQL to check if a subquery returns any rows. It evaluates to TRUE if the subquery returns at least one row and FALSE otherwise. Unlike JOIN or IN, the EXISTS clause does not return specific data from the subquery; it merely tests for the presence of rows.

SELECT product_name 
FROM sales 
WHERE EXISTS (SELECT * FROM returns WHERE returns.product_id = sales.product_id);

Here, the EXISTS clause ensures the product from the sales table has a matching product_id in the returns table. The SELECT 1 is used to signify the presence of rows without retrieving specific data.

41. What is a TEMPORARY table in SQL?

A TEMPORARY table is a table that exists temporarily during a session. It is automatically dropped when the session ends.

CREATE TEMPORARY TABLE temp_sales AS SELECT * FROM sales WHERE region = 'North';

42. What distinguishes an INNER JOIN from a LEFT JOIN? 

SQL joins are used to combine data from two or more tables by matching rows based on a common column. Two of the most commonly used joins are INNER JOIN and LEFT JOIN.   

• INNER JOIN: Retrieves only the rows that have corresponding matches in both tables. It focuses on matched rows only and provides context and additional information about the data.

Example

INNER JOIN

SELECT customers.name, orders.order_date FROM customers 
INNER JOIN orders ON customers.id = orders.customer_id;

• LEFT JOIN: Returns all rows from the left table, and the matched records from the right table. When there is no match in the right table, the corresponding columns from the right table will contain NULL values in the result.

LEFT JOIN

SELECT customers.name, orders.order_date FROM customers 
LEFT JOIN orders ON customers.id = orders.customer_id;

43. What are the various categories of SQL statements?

The four major types of SQL statements are:

• DDL (Data Definition Language): It defines database structures (e.g., CREATE, ALTER, DROP).
• DML (Data Manipulation Language): It manages data (e.g., SELECT, INSERT, UPDATE, DELETE).
• DCL (Data Control Language): It controls access (e.g., GRANT, REVOKE).
• TCL (Transaction Control Language): It manages transactions (e.g., COMMIT, ROLLBACK).

44. How do UNION and UNION ALL differ from each other?

The main difference between UNION and UNION ALL is how they handle duplicate rows in the result set.

45. What are subqueries, both nested and correlated?

Nested Subquery: A subquery placed inside another query, executed independently, and its result is used by the outer query.

 A correlated subquery is a subquery that references columns from the outer query and is executed repeatedly for each row processed by the outer query, making it dependent on the outer query's data.

Example of nested subquery:

SELECT product_name FROM products WHERE price > (SELECT AVG(price) FROM products);

The subquery calculates the average price of all products, and the outer query retrieves products priced higher than the average.

Example of correlated subquery:

SELECT name, salary FROM employees e
WHERE salary > (SELECT AVG(salary) FROM employees WHERE department = e.department);

The subquery calculates the average salary for each department, and the outer query retrieves employees earning more than the average in their department

Use Cases:

• Nested Subqueries: Ideal for comparisons or filtering based on static results.
• Correlated Subqueries: Used for row-by-row comparisons or calculations.

Limitations:

• Correlated subqueries can be inefficient for large datasets due to repeated execution.

In conclusion, SQL is a vital skill for any data analyst. As a powerful tool for querying and manipulating data, it allows data analysts to extract meaningful insights from complex datasets. By mastering SQL, data analysts can significantly improve the efficiency of their analysis and reporting tasks. The SQL interview questions discussed here cover fundamental concepts that employers typically ask to evaluate a candidate’s expertise. Preparing for these questions will help candidates confidently demonstrate their proficiency in SQL during interviews.

1. How can I prepare for SQL technical interview questions for data analysis?

Learn the above questions with answers and practice by visualizing real-world scenarios, such as data extraction, transformation, and reporting, which will help you prepare for SQL technical interview questions for data analysts. Focusing on efficiency, problem-solving, and query optimization is key.

2. What should I expect in SQL interview questions for analytics roles?

Expect questions on writing complex queries, aggregating data, performing calculations, and working with large datasets. Understanding how to optimize queries for large databases is crucial for SQL analytics interview questions.

3. What is the difference between SQL interview questions for data analyst freshers and experienced candidates?

For fresher candidates, the focus is on fundamental SQL concepts, such as SELECT queries, basic joins, and data filtering. For experienced candidates, SQL interview questions for experienced data analyst roles will likely be more complex, involving optimization techniques, advanced joins, and query performance tuning.