Ashish Yadav
Are you preparing for an advanced SQL interview? Look no further! In this blog post, we will be diving into some of the toughest SQL interview questions and providing in-depth explanations and sample answers to help you ace your interview.
SQL (Structured Query Language) is a programming language used to manage and manipulate relational databases. It is used to insert, update, and query data in a database. SQL commands include SELECT, FROM, WHERE, and JOIN, among others. It is a standard language for interacting with relational databases and is widely used in the industry.
Q1. What is SQL and why is it used?
Answer: SQL stands for Structured Query Language and is a programming language used to manage and manipulate data stored in relational databases. It is used to create, modify, and query databases.
Q2. What is a primary key?
Answer: A primary key is a column or set of columns in a table that uniquely identifies each row in the table. Primary keys cannot contain NULL values and must be unique across all rows in the table.
Q3. What is a foreign key?
Answer: A foreign key is a column or set of columns in a table that references the primary key in another table. It is used to establish a relationship between the two tables.
Q4. What is a JOIN and how many types of JOINs are there?
Answer: A JOIN is a way to combine rows from two or more tables based on a related column between them. There are several types of JOINs: INNER JOIN, OUTER JOIN (LEFT JOIN, RIGHT JOIN, FULL JOIN), and CROSS JOIN.
Q5. What is a UNION and how is it different from a JOIN?
Answer: UNION is used to combine the results of two or more SELECT statements into a single result set. It is different from a JOIN because it does not require a common column between the tables being combined.
Q6. What is a NULL value and how is it different from a zero or a blank space?
Answer: NULL is a special value that represents the absence of a value or the unknown. It is different from a zero or a blank space because it is a separate value that is distinct from all other values.
Q7. What is a database?
Answer: A database is a collection of data that is organized and stored in a structured format. It is used to store and manage large amounts of data and is typically accessed using a database management system (DBMS).
Q8. What is a database management system (DBMS)?
Answer: A database management system (DBMS) is a software application that allows users to create, manage, and manipulate databases. It provides a set of tools and functions for storing, organizing, and accessing data stored in a database.
Q9. What is a table in a database?
Answer: A table in a database is a structured collection of data stored in rows and columns. Tables are used to store data in a logical and organized way and can be used to represent real-world objects or concepts, such as customers or orders.
Q10. What is a primary key in a table?
Answer: A primary key is a field in a table that is used to uniquely identify each record in the table. It must contain unique values and cannot be null.
Q11. What is a foreign key in a table?
Answer: A foreign key is a field in a table that is used to establish a relationship with another table. It refers to the primary key of the related table and is used to enforce referential integrity.
Q12. How do you select data from a table in SQL?
Answer: You can use the SELECT statement to retrieve data from a table in SQL.
For example:
SELECT * FROM Customers;
This query will select all data from the Customers table. You can also specify specific columns to select, and use a WHERE clause to filter the results.Q13. How do you insert data into a table in SQL?
Answer: You can use the INSERT INTO statement to insert data into a table in SQL. For example:
INSERT INTO Customers (CustomerName, ContactName, Address)
VALUES ('Company Inc.', 'John Doe', '123 Main Street');
This query will insert a new record into the Customers table with the specified values for the CustomerName, ContactName, and Address columns.Q14. How do you update data in a table in SQL?
Answer: You can use the UPDATE statement to modify data in a table in SQL.
For example:
UPDATE Customers
SET ContactName = 'Jane Doe'
WHERE CustomerID = 1;
This query will update the ContactName field for the record with a CustomerID of 1 in the Customers table.Q15. How do you delete data from a table in SQL?
Answer: You can use the DELETE FROM statement to remove data from a table in SQL.
For example:
DELETE FROM Customers
WHERE CustomerID = 1;
This query will delete the record with a CustomerID of 1 from the Customers table.Q16. How do you retrieve all rows from a table in a database?
Answer: You can use the following SELECT statement to retrieve all rows from a table in a database:
SELECT * FROM table_name;Q17. How do you retrieve a specific number of rows from a table in a database?
Answer: You can use the LIMIT clause in a SELECT statement to retrieve a specific number of rows from a table in a database:
SELECT * FROM table_name LIMIT 10;
This will retrieve the first 10 rows from the table.Q18. How do you sort the results of a SELECT statement in ascending or descending order?
Answer: You can use the ORDER BY clause in a SELECT statement to sort the results in ascending or descending order. To sort in ascending order, use the following syntax:
SELECT * FROM table_name ORDER BY column_name ASC;
To sort in descending order, use the following syntax:
SELECT * FROM table_name ORDER BY column_name DESC;Q19. How do you insert a new row into a table in a database?
Answer: You can use the INSERT INTO statement to insert a new row into a table in a database:
INSERT INTO table_name (column_1, column_2, …) VALUES (value_1, value_2, …);Q20. How do you update the values in a row in a table in a database?
Answer: You can use the UPDATE statement to update the values in a row in a table in a database:
UPDATE table_name SET column_1 = value_1, column_2 = value_2, … WHERE condition;Q21. How would you find the second-highest salary of an employee in a table?
Answer:
SELECT salary FROM employees
WHERE salary NOT IN (SELECT MAX(salary) FROM employees)
ORDER BY salary DESC
LIMIT 1
How would you find the most common values in a table?
Copy code
SELECT value, COUNT(*) AS count
FROM table
GROUP BY value
ORDER BY count DESC
LIMIT 1Q22. How would you find the second most common values in a table?
Answer:
SELECT value, COUNT(*) AS count
FROM table
GROUP BY value
ORDER BY count DESC
LIMIT 1, 1Q23. How would you find the third most common value in a table?
Answer:
SELECT value, COUNT(*) AS count
FROM table
GROUP BY value
ORDER BY count DESC
LIMIT 2, 1Q24. How would you find the top 10% of salaries in a table?
Answer:
SELECT salary FROM employees
WHERE salary IN (SELECT salary FROM employees
ORDER BY salary DESC
LIMIT (SELECT CEILING(COUNT(*) * 0.1) FROM employees))Q25. How would you find the number of employees in each department?
Answer:
SELECT department, COUNT(*) AS num_employees
FROM employees
GROUP BY departmentQ26. How would you find the average salary of employees in each department?
Answer:
SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY departmentQ27. How would you find the maximum and minimum salary of employees in each department?
Answer:
SELECT department, MAX(salary) AS max_salary, MIN(salary) AS min_salary
FROM employees
GROUP BY departmentQ28. How would you find the names of employees who have the highest salary in their department?
Answer:
SELECT name, salary, department
FROM employees e1
WHERE salary = (SELECT MAX(salary) FROM employees e2 WHERE e1.department = e2.department)Conclusion:
With a solid understanding of these advanced SQL concepts and the ability to effectively communicate your knowledge through clear and concise answers, you will be well-prepared for any SQL-related interview. Best of luck with your interview!
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