If you are preparing for your next data analyst interview questions and looking for essential SQL questions, then you’re at the perfect place. In this blog, I will share 11 important SQL interview questions for Data Analysts along with their answers.
What is SQL?
Structured Query Language (SQL) is a powerful and standardized programming language designed for managing and manipulating relational databases. SQL can be used as a fundamental tool for extracting, organizing, and manipulating data.
Essential SQL Concepts
SQL provides a set of commands and syntax for interacting with relational database management systems (RDBMS). These systems are utilized to store and retrieve data in a structured and well-organized manner. SQL enables users to define, query, and manipulate the data within these databases seamlessly.
SQL is composed of various commands, each serving a specific purpose. The most common SQL clauses include:
- SELECT: To Retrieve data from one or more tables in database.
- INSERT: DML command used to add new records to a table.
- UPDATE: Modifies existing records in a table.
- DELETE: Removes records from a table.
- CREATE: DDL command used to build new database objects such as tables, views, or indexes.
- ALTER: DDL command used to modify the structure of an existing database object.
- JOIN: Used in a query to combine data from multiple tables based on specified relationships.
Why Data Analysts Use SQL?
SQL is well-known for its simplicity and declarative nature, allowing users to express specifically what data they want rather than specifying how to retrieve it. This makes it an invaluable tool for data analysts, as it facilitates efficient data manipulation, extraction, and reporting.
Data Analysts consistently rely on SQL to uncover meaningful insights from vast datasets, perform complex aggregations, and generate reports that drive informed decision-making. Its ubiquity across various database systems and its standardized syntax make it an indispensable skill in the realm of data analytics.
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11 Most Important SQL Interview Questions For Data Analysts
Q.1- How do you find duplicate values in a table using SQL?
Q.2- How do you delete duplicates from a table using SQL?
Q.3- Tell me difference between union and union all in SQL?
Q.4- Tell me difference between rank, row_number and dense_rank in SQL?
Q.5- How will you find records in a table which are not present in another table using SQL?
Q.6- How will you find the second highest salary employees in each department using SQL?
Q.7- How will you find employees with salary more than their manager’s salary using SQL?
Q.8- What is the difference between inner and left join in SQL?
Q.9- How will you update a table to swap gender values using SQL?
Q.10- Assume you have two tables in a database – one named “Orders” containing information about customer orders, and another named “Customers” with details about the customers. The common field between these tables is “CustomerID.” Your task is to retrieve a list of all orders with the corresponding customer information (customer name, order date, and product) for a given date range (let’s say, between ‘2023-01-01’ and ‘2023-12-31’).
Q.11- Consider a table named “Sales” that contains information about product sales, including columns such as “ProductID,” “SaleDate,” and “QuantitySold.” The task is to write a SQL query to find the total quantity sold for each product, along with the product name, and present the results in descending order of total quantity sold.
Think for a moment to figure out answer for each question on your own before scrolling down for the correct answers.
Answers for the SQL Interview Questions For Data Analysts
1- To find duplicates in a table, you can use the following SQL query:
SELECT column_name, COUNT(*)
GROUP BY column_name
HAVING COUNT(*) > 1;
2- Let’s think step-by-step to delete duplicates from a table in SQL. There are two ways to tackle this question. Clarify with the interviewer if they want to keep one instance of duplicate row or delete all duplicate rows. Here is the SQL code to remove all duplicate rows.
DELETE FROM table_name
WHERE column_name IN (
GROUP BY column_name
HAVING COUNT(*) > 1
Explanation: The given SQL snippet is a statement that deletes duplicate records from a database table. Here’s a breakdown of the code:
DELETE FROM table_name: This part of the statement specifies the table from which the records will be deleted.
WHERE column_name IN (SELECT column_name FROM table_name GROUP BY column_name HAVING COUNT(*) > 1): This part of the statement uses a subquery to identify the duplicate records. It selects the column_name from the table, groups the records by the column_name, and then filters the groups to only include those with a count greater than 1. The IN keyword is used to match the records in the table with the results of the subquery, and only the duplicate records will be selected for deletion.
If you want to keep one instance of each duplicate row and delete the others, you can use the following SQL query:
FROM table_name t1
JOIN table_name t2 ON t1.column_name = t2.column_name
WHERE t1.id > t2.id;
This query deletes duplicates by keeping the one with the lower
id value. Adjust the column names and conditions based on your specific table structure and requirements.
3- The main difference between
UNION ALL is that
UNION removes duplicate rows, while
UNION ALL includes all rows, even if they are duplicates.
4- In SQL,
DENSE_RANK() are used for ranking.
RANK() leaves gaps for tied ranks,
ROW_NUMBER() assigns a unique number to each row, and
DENSE_RANK() leaves no gaps for tied ranks.
5- To find records in a table that are not present in another table, you can use the
NOT EXISTS or
LEFT JOIN with
NULL check. Example:
WHERE NOT EXISTS (
WHERE table1.id = table2.id
6- To find the second highest salary employees in each department, you can use a subquery with the
ROW_NUMBER() function. Example:
SELECT employee_id, department_id, salary
SELECT employee_id, department_id, salary,
ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rn
) AS ranked
WHERE rn = 2;
This SQL query is used to retrieve the employee ID, department ID, and salary of the second-highest paid employee within each department. Here’s a breakdown of the code:
- The inner SELECT statement calculates the row number (rn) over each department’s employees based on their salaries, in descending order. It assigns the row number 1 to the highest-paid employee, 2 to the second-highest paid employee, and so on.
- The outer SELECT statement filters the results from the inner query to return only the records where the row number (rn) is 2, indicating the second-highest paid employee in each department.
In summary, this query fetches the employee ID, department ID, and salary of the second-highest paid employee in each department from the “employees” table.
7- To find employees with salary more than their manager’s salary, you can use a self-join. Example:
SELECT e.employee_id, e.employee_name, e.salary
FROM employees e
JOIN employees m ON e.manager_id = m.employee_id
WHERE e.salary > m.salary;
8- The main difference between inner join and left join is that an inner join returns only the matching rows in both tables, while a left join returns all rows from the left table and the matching rows from the right table.
9- To update a table and swap gender values, you can use a temporary variable to store one of the values temporarily. Example:
SET gender = CASE
WHEN gender = 'Male' THEN 'Female'
WHEN gender = 'Female' THEN 'Male'
10- TO Retrieve Specific Data from Multiple Tables:
Customers ON Orders.CustomerID = Customers.CustomerID
Orders.OrderDate BETWEEN '2023-01-01' AND '2023-12-31';
In this query, we’re selecting the order date, customer name, and product from the “Orders” and “Customers” tables. The JOIN clause is used to combine the data based on the common field “CustomerID,” and the WHERE clause is used to filter the results for the specified date range.
11- To Aggregate Data for Analysis:
SUM(Sales.QuantitySold) AS TotalQuantitySold
Products ON Sales.ProductID = Products.ProductID
This query aggregates data from the “Sales” table, calculating the total quantity sold for each product. The JOIN clause links the “Sales” table with the “Products” table using the common field “ProductID.” The GROUP BY clause groups the results by product name, and the ORDER BY clause arranges the output in descending order of total quantity sold.
Alright! so these were some tricky SQL Interview Questions For Data Analysts. Please let me know in case if i missed something or you want to add anything in this blog.
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