Understanding SQL interview questions, especially those related to query optimization, can enhance skills in providing faster outputs and executing a larger number of queries in less time!
SQL Interview Questions: Stepping into the world of data science can be as thrilling as it is daunting. “The most important thing in a data science career is to have a good mentor,” echoes across the industry, speaking to the heart of every aspiring data analyst and engineer who has faced the formidable task of prepping for an SQL interview.Â
Whether you’re a fresh graduate, a seasoned veteran polishing your skills, or somewhere in between, having the right resources can make all the difference. This SQL Interview Questions CHEAT SHEET for 2024 is your secret weapon, crafted to turn the tide in your favor, illuminating complex concepts and common queries you’re destined to encounter.
So, let’s embark on this journey together—where each question unveils a fragment of the vast universe that is SQL, preparing you not just to answer with confidence, but to inspire awe with your depth of understanding.
Whether you’re a job seeker seeking to apply your SQL skills to new opportunities or a hiring manager preparing to interview candidates for a job opening in your company, it’s crucial to be well-versed in common SQL interview questions and their corresponding answers.
SQL Interview Questions MCQ
Here are some multiple-choice questions (MCQs) related to SQL:
1) What does SQL stand for?
- a) Simple Query Language
- b) Structured Question Language
- c) Standardized Query Language
- d) Structured Query Language
Answer: d) Structured Query Language
2) Which SQL statement is used to retrieve data from a database?
- a) SELECT
- b) RETRIEVE
- c) GET
- d) EXTRACT
Answer: a) SELECT
3) In SQL, what is the purpose of the WHERE clause?
- a) To filter the result set based on conditions
- b) To specify the order of the result set
- c) To join multiple tables
- d) To group the result set
Answer: a) To filter the result set based on conditions
4) Which SQL command is used to insert new data into a database?
- a) UPDATE
- b) INSERT
- c) ADD
- d) MODIFY
Answer: b) INSERT
5) What is the primary key in a relational database?
- a) A key that is used for encryption
- b) A key used to unlock tables
- c) A unique identifier for a record in a table
- d) A key used to establish relationships between tables
Answer: c) A unique identifier for a record in a table
6) Which SQL clause is used to sort the result set?
- a) SORT BY
- b) ORDER BY
- c) GROUP BY
- d) SORT
Answer: b) ORDER BY
7) What type of join returns all rows when there is a match in one of the tables?
- a) INNER JOIN
- b) LEFT JOIN
- c) RIGHT JOIN
- d) FULL JOIN
Answer: a) INNER JOIN
8) Which SQL function is used to find the total number of rows in a table?
- a) COUNT(*)
- b) SUM()
- c) AVG()
- d) TOTAL()
Answer: a) COUNT(*)
9) What does the SQL acronym “ACID” stand for in the context of database transactions?
- a) Atomicity, Consistency, Isolation, Durability
- b) Association, Continuity, Integration, Dependability
- c) Aggregation, Compatibility, Independence, Durability
- d) Accuracy, Control, Integrity, Durability
Answer: a) Atomicity, Consistency, Isolation, Durability
10) Which SQL command is used to remove all data from a table without removing the table itself?
- a) REMOVE
- b) TRUNCATE
- c) DELETE
- d) DROP
Answer: b) TRUNCATE
These multiple-choice questions cover various aspects of SQL and are designed to test your knowledge of fundamental SQL concepts.
SQL Interview Questions PDFÂ
SQL interview questions PDF available on Physics Wallah, covering real-world SQL interview questions for 2024. The document is a valuable resource for preparation, offering insights into commonly asked questions in SQL interviews.
The PDF encompasses a wide range of topics, including DBMS, SQL queries, and essential SQL concepts. Candidates can use this comprehensive guide to enhance their understanding and readiness for SQL interviews.
PW offers a comprehensive guide with the “Top SQL Interview Questions and Answers for 2024,” providing additional insights and tips for successful SQL interviews.
Tricky SQL Queries for InterviewÂ
Here are some tricky SQL queries that are commonly used in interviews:
1) Find the Second Highest Salary:
SELECT MAX(salary) AS SecondHighestSalary
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);
2) Get Nth Highest Salary:
SELECT DISTINCT salary
FROM employees e1
WHERE N = (SELECT COUNT(DISTINCT salary)Â
 FROM employees e2Â
 WHERE e1.salary <= e2.salary);
3) Calculate Running Total:
SELECT date, amount,Â
 SUM(amount) OVER (ORDER BY date) AS RunningTotal
FROM transactions;
4) Pivot Table:
SELECTÂ
 MAX(CASE WHEN product = ‘A’ THEN revenue END) AS A_Revenue,
 MAX(CASE WHEN product = ‘B’ THEN revenue END) AS B_Revenue,
 MAX(CASE WHEN product = ‘C’ THEN revenue END) AS C_Revenue
FROM sales;
5) Find Duplicate Rows:
SELECT column1, column2, COUNT(*)
FROM table_name
GROUP BY column1, column2
HAVING COUNT(*) > 1;
6) Ranking with Ties:
SELECT emp_name, salary,
 DENSE_RANK() OVER (ORDER BY salary DESC) AS Rank
FROM employees;
7) Generate Fibonacci Sequence:
SELECT n,Â
 ROUND((POWER((1 + SQRT(5)) / 2, n) – POWER((1 – SQRT(5)) / 2, n)) / SQRT(5)) AS Fibonacci
FROM numbers;
8) Find Missing Number in Sequence:
SELECT DISTINCT (a.id + 1) AS MissingNumber
FROM numbers a
LEFT JOIN numbers b ON a.id + 1 = b.id
WHERE b.id IS NULL;
These queries cover a range of SQL concepts and are designed to assess problem-solving skills during interviews.
SQL Interview Questions for Data AnalystÂ
Here are some SQL interview questions that are commonly asked in interviews for Data Analyst positions:
Basic SQL Questions:
1) What is SQL?
- SQL stands for Structured Query Language. It is a domain-specific language used to manage and manipulate relational databases.
2) Explain the difference between SQL and MySQL.
- SQL is a language used to manage and manipulate relational databases, while MySQL is a relational database management system (RDBMS) that uses SQL as its query language.
3) What is a database?
- A database is a structured collection of data organized in a way that a computer program can quickly select and retrieve specific pieces of data.
4) What is a primary key?
- A primary key is a unique identifier for a record in a database table. It must contain unique values and cannot have NULL values.
5) What is the difference between CHAR and VARCHAR data types?
- CHAR is a fixed-length character data type, while VARCHAR is a variable-length character data type. CHAR will always reserve the specified length, while VARCHAR only stores the actual characters entered.
Intermediate SQL Questions:
1) Write an SQL query to retrieve all the columns from a table named “employees.”
SELECT * FROM employees;Â
2) What is the purpose of the GROUP BY clause in SQL?
- The GROUP BY clause is used to group rows that have the same values in specified columns into summary rows.
3) Explain the difference between INNER JOIN and LEFT JOIN.
- INNER JOIN returns only the rows where there is a match in both tables, while LEFT JOIN returns all rows from the left table and the matched rows from the right table.
4) Write an SQL query to calculate the average salary of employees in a table named “salaries.”
SELECT AVG(salary) FROM salaries;Â
5) Explain the purpose of the HAVING clause in SQL.
- The HAVING clause is used in combination with the GROUP BY clause to filter the results of aggregate functions based on specified conditions.
Advanced SQL Questions:
1) What is a subquery?
- A subquery is a query nested within another query. It can be used to retrieve data that will be used in the main query as a condition.
2) Write an SQL query to find the second-highest salary from a table named “employees.”
SELECT MAX(salary) FROM employees WHERE salary < (SELECT MAX(salary) FROM employees);Â
3) Explain the concept of indexing in databases.
- Indexing is a database optimization technique used to speed up the data retrieval process by creating a data structure (index) that allows quicker access to rows.
4) What is a stored procedure?
- A stored procedure is a set of SQL statements that can be stored in the database and executed later. It enhances the reusability and performance of database operations.
5) Write an SQL query to find the nth highest salary from a table named “salaries.”
SELECT salary FROM salaries ORDER BY salary DESC LIMIT 1 OFFSET n-1;Â
(Replace ‘n’ with the desired rank, e.g., 2nd highest salary)
Also Read: Top 30+ Angular Interview Questions and Answers for 2024
MySQL Interview QuestionsÂ
Here are some MySQL interview questions that cover a range of concepts:
Basic MySQL Interview Questions:
1) What is MySQL?
MySQL is an open-source relational database management system (RDBMS) that uses SQL (Structured Query Language) for managing and manipulating the data in the database.
2) Explain the difference between MyISAM and InnoDB storage engines.
MyISAM is a non-transactional storage engine, while InnoDB is a transactional storage engine. InnoDB supports ACID properties and is more suitable for applications that require transactions and data integrity.
3) How do you create a new database in MySQL?
CREATE DATABASE database_name;
4) What is the purpose of the MySQL SELECT statement?
The SELECT statement is used to retrieve data from one or more tables in a MySQL database.
5) How do you retrieve all columns from a table named “employees”?
SELECT * FROM employees;
Intermediate MySQL Interview Questions:
1) Explain the concept of MySQL Index and its types.
An index in MySQL is a data structure that improves the speed of data retrieval operations on a database table. Types of indexes include PRIMARY KEY, UNIQUE, INDEX, and FULLTEXT.
2) Write a MySQL query to update the salary of an employee with the ID of 101.
UPDATE employees SET salary = 60000 WHERE employee_id = 101;
3) What is a MySQL Stored Procedure?
A stored procedure is a set of SQL statements that can be stored in the MySQL database and executed with a single call.
4) Explain the purpose of the MySQL JOIN clause.
The JOIN clause is used to combine rows from two or more tables based on a related column between them.
5) How do you find the total number of rows in a table in MySQL?
SELECT COUNT(*) FROM table_name;
Advanced MySQL Interview Questions:
1) Explain the difference between INNER JOIN and LEFT JOIN.
INNER JOIN returns only the matching rows between the tables, while LEFT JOIN returns all rows from the left table and the matching rows from the right table.
2) What is MySQL partitioning, and when is it used?
Partitioning in MySQL involves splitting large tables into smaller, more manageable pieces. It is used to improve query performance and manageability.
3) Write a MySQL query to find the second-highest salary from an “employees” table.
SELECT MAX(salary)Â
FROM employeesÂ
WHERE salary < (SELECT MAX(salary) FROM employees);
4) Explain the purpose of the MySQL TRIGGER.
A trigger in MySQL is a set of instructions that are automatically executed (“triggered”) in response to certain events on a particular table or view.
5) How do you perform a backup and restore in MySQL?
To perform a backup:
mysqldump -u username -p database_name > backup.sql
To perform a restore:
mysql -u username -p database_name < backup.sql
These MySQL interview questions cover both fundamental and advanced concepts that are relevant for candidates with various levels of experience working with MySQL databases.
Also Read: 10 Online Computer Programming Courses to Enroll in 2024
SQL Interview Questions for 5 Years Experience
Here are some SQL interview questions suitable for candidates with around 5 years of experience:
Basic SQL Questions:
1) What is normalization, and why is it important in databases?
- Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves breaking down large tables into smaller, related tables. It is essential for minimizing data duplication and ensuring consistency.
2) Explain the differences between UNION and UNION ALL in SQL.
- UNION combines the result sets of two SELECT statements, removing duplicates, while UNION ALL combines result sets without removing duplicates.
3) What is the purpose of the SQL WHERE clause?
- The WHERE clause is used to filter records based on specified conditions, allowing you to retrieve only the rows that meet certain criteria.
4) What is an SQL View?
- An SQL View is a virtual table based on the result of a SELECT query. It does not store the data itself but provides a way to represent complex queries or specific subsets of data.
5) Explain the concept of an SQL Index.
- An SQL Index is a data structure that improves the speed of data retrieval operations on a database table. It is created on one or more columns of a table to allow faster query execution by reducing the number of data pages that need to be scanned.
Intermediate SQL Questions:
1) Write an SQL query to find the third-highest salary from an “employees” table.
SELECT MAX(salary) FROM employees WHERE salary NOT IN (SELECT MAX(salary) FROM employees UNION SELECT MIN(salary) FROM employees);Â
2) Explain the differences between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.
- INNER JOIN returns rows with matching values in both tables; LEFT JOIN returns all rows from the left table and matched rows from the right table; RIGHT JOIN does the opposite of LEFT JOIN, and FULL JOIN returns all rows when there is a match in either table.
3) What is the purpose of the SQL GROUP_CONCAT function?
- GROUP_CONCAT is used to concatenate values from multiple rows into a single string. It is often used with the GROUP BY clause.
4) Explain the ACID properties in the context of database transactions.
- ACID stands for Atomicity, Consistency, Isolation, and Durability. It ensures that database transactions are processed reliably. Atomicity ensures that transactions are treated as a single, indivisible unit; Consistency ensures that the database remains in a valid state before and after the transaction; Isolation ensures that multiple transactions can occur concurrently without interfering with each other; and Durability ensures that committed transactions persist even in the event of a system failure.
5) Write an SQL query to calculate the total number of orders for each customer from an “orders” table.
SELECT customer_id, COUNT(order_id) AS total_orders FROM orders GROUP BY customer_id;Â
Advanced SQL Questions:
1) Explain the purpose of the SQL CASE statement.
- The CASE statement is used to perform conditional logic in SQL queries. It allows you to return different values or perform different actions based on specified conditions.
2) What is a recursive SQL query, and when is it used?
- A recursive SQL query is a query that refers to itself. It is often used to query hierarchical data structures, such as organizational charts or tree structures.
3) Write an SQL query to find duplicate rows in a table.
SELECT column1, column2, COUNT(*) FROM table_name GROUP BY column1, column2 HAVING COUNT(*) > 1;Â
4) Explain the concept of database sharding.
- Database sharding is a database architecture strategy where a large database is divided into smaller, more manageable parts called shards. Each shard is an independent database and is responsible for a specific range of data. Sharding is used to improve scalability and performance.
5) What are SQL Window Functions? Provide an example.
- SQL Window Functions perform a calculation across a set of rows that are related to the current row. An example is the ROW_NUMBER() function, which assigns a unique number to each row within a partition of a result set.
SELECT employee_id, salary, ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank FROM employees;
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SQL Interview Questions FAQs
What is SQL?
SQL stands for Structured Query Language. It is a standard programming language used to manage and manipulate relational databases. SQL is used for tasks such as querying data, updating data, and creating and modifying database schemas.
What are the types of SQL statements?
There are mainly three types of SQL statements:
DDL (Data Definition Language): Used to define the database structure, such as creating, altering, and deleting tables.
DML (Data Manipulation Language): Used to manipulate the data stored in the database, such as inserting, updating, and deleting records.
DQL (Data Query Language): Used to query the data from the database, primarily using the SELECT statement.
Explain the difference between INNER JOIN and LEFT JOIN.
INNER JOIN: Returns only the rows where there is a match in both tables.
LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table and the matched rows from the right table. If there is no match, NULL values are returned for columns from the right table.
What is a primary key?
A primary key is a column or a set of columns that uniquely identifies each row in a table. It must contain unique values, and it cannot have NULL values.
Explain the ACID properties in the context of databases.
ACID stands for Atomicity, Consistency, Isolation, and Durability. These properties ensure the reliability of transactions in a database:
Atomicity: Transactions are treated as a single, indivisible unit of work. Either all changes are committed, or none are.
Consistency: Transactions bring the database from one valid state to another. The database must satisfy predefined integrity constraints.
Isolation: Each transaction is executed in isolation from other transactions. Intermediate states of a transaction are not visible to other transactions.
Durability: Once a transaction is committed, its changes are permanent and survive subsequent failures.
