SQL Interview Questions for Data Analysts in 2024

Summary: Prepare thoroughly for data analyst interviews with our comprehensive guide on SQL interview questions. Learn essential SQL concepts, practice example queries, and gain confidence in solving real-world data problems.

Introduction

Preparing for the SQL interview questions to become more proficient in data science with example questions and a framework for answering them.

SQL is a special tool that data analysts must have in their arsenal. Its tabular format for data storage makes it easier to comprehend the information. In addition, several other features of SQL make it one of the popular tools for data scientists. 

The blog includes tips and tactics for confidently responding to SQL screening interviews. It also covers a variety of SQL query-related questions, an example coding question, and the step-by-step process for writing SQL code during the interview.

Structured Query Language or SQL for Data Science is one of the most critical skills every Data Science aspirant must master. As you prepare for your interview for Data Science roles, you should also start grasping the concepts of SQL to come up with flying colours.

Read Blog: Differences Between SQL and T-SQL [with Example].

General SQL Interview Composition 

SQL is essential for information retrieval, analysis, and interpretation. Anyone working in Data Science uses it to interface with relational databases.

As an application of SQL in Data Science is indispensable, every aspiring data scientist preparing for this interview should be well-versed in it. Most data scientist or data analyst interviews have a SQL technical evaluation section. This phase of the interview demands that you address real-world problems using SQL.

Usually, you may be given some typological issues. Still, the ultimate purpose is for the interviewer to check if you can utilise SQL rather than merely talking about it. So, knowing the terms and solving problems in SQL is necessary for cracking any interview. Most commonly, such exams take one of three forms:

• Whiteboard: SQL whiteboard exams are a regular component of interviews. A whiteboard test requires you to construct SQL queries by hand. This allows firms to check your comprehension of SQL principles and problem-solving abilities.
• Coding tests: Many employers may ask you to develop code and execute queries during live interviews. You may check for syntax problems while working with live code displays, allowing firms to monitor your coding effectiveness.
• SQL case studies: In the case of interviews, you are given a real-world problem and asked to answer it using your SQL expertise. These are often open-ended inquiries that allow for analysis and problem-solving creativity.

Also Check: Advanced SQL Tips and Tricks for Data Analysts.

Some Common SQL Questions

Understanding common SQL questions is crucial for anyone working with databases. It helps master query writing, optimise database performance, and ace interviews. Familiarity with SQL questions enhances problem-solving skills and ensures efficient data retrieval and manipulation, making it indispensable in data-driven professions.

1. What is a Primary Key?

In SQL, a primary key is a distinct identifier for each record (row) in a table. It guarantees that no two rows have identical values in the primary key fields and enforces the integrity of the data in the table.

A primary key in a table can be one or many columns. A composite primary key is created when numerous columns are utilised. Primary fundamental column values must be unique across all rows in the table and cannot be NULL. Foreign keys in linked tables are derived from primary keys.

2. What are the different subsets of SQL?

SQL (Structured Query Language) is divided into numerous subsets that serve distinct purposes:

• DDL (Data Definition Language): This SQL subset comprises instructions to define the database structure, such as CREATE, ALTER, and DROP statements.
• Data Manipulation Language (DML): This subset of SQL comprises instructions for managing and manipulating data stored in a database, such as SELECT, INSERT, UPDATE, and DELETE statements.
• Data Control Language (DCL): This SQL subset comprises commands that control database and data access, such as the GRANT and REVOKE statements.
• Transaction Control Language (TCL): This SQL subset comprises transaction-management commands such as the COMMIT, ROLLBACK, and SAVEPOINT statements.
• Dynamic SQL: It is a subset of SQL that contains instructions for dynamically generating and executing SQL statements at runtime.

3. What do you mean by DBMS? What are its different types?

DBMS is an abbreviation for Database Management System. It is a software system that allows users to save, retrieve, and change data by providing an interface for dealing with a database.

There are several types of DBMS, including:

• Relational DBMS (RDBMS): A type of DBMS that organises data into tables, with each table consisting of rows (records) and columns (fields). Relationships between tables can be established using keys.
• Object-Relational DBMS (ORDBMS): A DBMS that extends the relational model by including support for objects and object-oriented programming concepts.
• Hierarchical DBMS: A type of DBMS that organises data into a tree-like structure, with each record having one parent and zero or more children.
• Network DBMS: A DBMS that organises data into a network of records, where records can have multiple parent and child relationships.
• NoSQL DBMS: A type of DBMS that does not use the relational model and instead uses alternative data storage and retrieval methods, such as a key-value, document-based, graph-based, or column-based.
• Distributed DBMS: A DBMS that distributes data and processing across multiple physical and/or logical servers, allowing for scalability and improved performance.

4. What are the most common aggregate functions in SQL? What do they do?

An aggregate function calculates a set of values and returns a single result that sums the set. SQL’s most common aggregate functions are COUNT, SUM, and AVG.

• COUNT: It returns the number of items in a group.
• SUM: This function returns the sum of ALL or DISTINCT values in an expression.
• AVG: It returns the average of values in a group (and ignores NULL values).

5. What is the difference between an Inner, Outer,  Right, and Left join?

Here are the different types of JOINs in SQL:

• INNER JOIN: It returns records that have matching values in both tables
• LEFT JOIN: It returns all records from the left table and the matched records from the right table
• RIGHT JOIN: It returns all records from the right table and the matched records from the left table
• FULL JOIN: It returns all records when there is a match in either the left or right table

6. What command would you use to update data in a table?  

In SQL, the UPDATE command is used to modify existing records in a table. It is combined with SET, which specifies the columns to be modified along with their new values and WHERE to select the rows that will be updated based on certain conditions.

Example: Suppose we have a table named Customer and want to update the ContactName for a customer with CustomerID 1 to “Ashutosh Jindal”.

7. Which operator is used to select values within a range? 

The BETWEEN operator selects values within a specified range, which includes both the start and end values.

Example: To select CustomerID values between 378 and 492 from the Customer table:

8. When would you use the GROUP BY statement?

The GROUP BY statement in SQL groups rows with the same values in specified columns into summary rows, typically for calculating aggregate values such as the sum, average, count, etc., for each group.

It is commonly used in combination with aggregate functions such as SUM(), AVG(), COUNT(), MIN(), MAX(), etc., to perform data analysis. 

Example: To calculate the total sales for each product in a sales_table, you might use:

This query aggregates sales by product_id, showing the total sales for each unique product in the sales_table.

9. What is a Self-Join?

A self-join in SQL is when the table is joined to itself. In other words, a self-join allows you to compare rows within a single table. A self-join is helpful in cases where you want to find relationships between rows within the same table, 

For example, I found all employees who are managers of other employees in an ‘EMPLOYEES’ table. To do this, you would join the ‘EMPLOYEES’ table with itself, using a ‘WHERE’ clause to specify the relationship between the two instances of the table.

10. What is schema in SQL Server?

A schema in SQL Server is a container for database objects such as tables, views, stored procedures, and functions. Schemas provide a way to logically group these objects within a database, making it easier to manage and organise the database structure.

A schema can be considered a namespace for database objects, similar to how a folder is a namespace for files in a file system. Each database object within a schema has a unique name but can have the same name as an object in another schema.

In SQL Server, each database user can own one or more schemas. By default, each user is assigned a default schema with the same name as the user.

Example Coding Question (Real industry based)

Question: Analyse the best days for displaying ads on different platforms for Diwali Sales Analysis

To analyse the best days for displaying ads for Diwali sales on different platforms using SQL, you would need to have data about the sales performance on each platform for previous Diwali seasons and the sales performance on each platform for the current Diwali season.

Assuming you have this data in a table named sales, you could perform the following analysis:

In this example, the ‘WITH’ clause creates a derived table ‘diwali_sales’ that aggregates the sales data for each platform and the date for the Diwali season. The main ‘SELECT’ statement then selects the platform, date, total sales, and a ranking based on the total sales for each platform. 

The ‘ROW_NUMBER()’ function generates the ranking, with the ‘PARTITION BY’ clause specifying that the ranking should be separate for each platform and the ‘ORDER BY’ clause specifying that it should be based on total sales.

The result of this query will be a table with columns for the platform, date, total sales, and rank. You can then use this table to identify the best days for displaying ads on each platform by selecting the rows with the highest rank.

Further Read: 

How do you drop a database on an SQL server?

Unlocking the Power of Rank Function: Your Guide to SQL Ranking.

SQL Server Error 26 and Methods to Resolve It.

Wrapping It Up

This brief guide on SQL will help you prepare to excel in Data Science interviews.  Even if you have just started with SQL, regular practice and persistence will help you excel. Moreover, try to answer as many SQL interview questions as possible. 

At Pickl.AI, we assist you in mastering all such integral data science concepts. Pickl.AI assures you that by the end of its Data Science Program, you will excel in all the conceptual and practical implications of Data Science, from case studies to projects and internships.