Top SQL Interview Questions and Answers

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Top SQL Interview Questions and Answers

Structured Query Language (SQL) is an essential programming language that enables users to manage and manipulate relational databases. It is widely used by organisations of all sizes to store, manage, and retrieve data efficiently. SQL skills are in high demand in the job market, and employers often assess candidates’ proficiency in SQL during interviews. 

If you are preparing for a SQL interview, it’s essential to know the commonly asked questions and their answers to increase your chances of success. 

In this article, we will highlight the top SQL interview questions and answers and provide comprehensive answers to help you prepare for your SQL interview. 

Basic Level SQL Interview Questions

  • What is SQL, and what purposes does it serve?

Structured Query Language is known as SQL. Relational databases are managed and manipulated using it.

  • Describe a database.

A database is a group of data that has been structured and arranged for easy retrieval and management.

  • What is the syntax to select all columns from a table in SQL?

To select all columns from a table in SQL, use the following syntax:

SELECT * FROM table_name;
  • What is the syntax to select specific columns from a table in SQL?

To select specific columns from a table in SQL, use the following syntax:

SELECT column1, column2, ... FROM table_name;
  • What is the syntax to join two tables in SQL?

To join two tables in SQL, use the following syntax:

SELECT column1, column2, ... FROM table1 JOIN table2 ON table1.column = table2.column;
  • What is a primary key in SQL?

A primary key is a column or set of columns in a table that uniquely identifies each row in the table. It’s used to enforce data integrity and ensure that there are no duplicate records.

  • What is a foreign key in SQL?

A foreign key is a column or set of columns in one table that refers to the primary key of another table. It’s used to create relationships between tables and enforce referential integrity.

  • What is a join in SQL?

A join is a way to combine data from two or more tables based on a common column. There are several types of joins, including inner join, left join, right join, and full outer join.

  • What does a SQL subquery do?

A query that is nested inside another query is known as a subquery. It is used to get data that will be incorporated into a bigger query.

SELECT *
FROM orders
WHERE customer_id IN (SELECT id FROM customers WHERE country = 'USA');
  • What does a SQL stored procedure do?

A stored procedure is a pre-written block of code that may be invoked by other applications or scripts and is kept in a database. It is employed to streamline challenging activities and enhance performance.

  • What is SQL normalisation?

The practice of structuring data in a database to eliminate redundancy and increase data integrity is known as normalisation. It entails breaking down data into smaller tables and establishing linkages between them.

  • What is SQL denormalization?

Denormalization is the deliberate introduction of redundancy into a database in order to increase efficiency. Data from numerous tables are combined into a single table.

  • What does SQL indexing mean?

The process of constructing an index makes it possible to retrieve data from a database more quickly. It entails building an index on one or more table columns.

  • What does a SQL transaction mean?

A series of database activities are grouped together into a transaction and handled as one workpiece. It is used to make sure that every action is successful or to undo any mistakes.

  • What does a SQL trigger do?

An exclusive class of stored procedures known as a trigger is carried out automatically in reaction to a certain event, such as a data update or insert.

  • What does a SQL constraint mean?

A constraint is a regulation that is applied to database data to preserve data integrity. Primary key, foreign key, and unique constraints are typical varieties of constraints.

  • What is the difference between UNION and UNION ALL in SQL?

UNION is used to combine the results of two or more SELECT statements into a single result set, while UNION ALL returns all rows from both SELECT statements, including duplicates.

  • What is the difference between WHERE and HAVING in SQL?

WHERE is used to filter rows based on a condition, while HAVING is used to filter groups based on a condition. HAVING is only used in conjunction with GROUP BY.

  • What is the difference between a clustered and non-clustered index in SQL?

A clustered index determines the physical order of data in a table, while a non-clustered index is a separate data structure that stores a copy of selected columns from a table.

  • What is a stored function in SQL?

A stored function is a pre-written block of code that can be called by other programs or scripts. It returns a single value and can be used in SQL statements just like a regular column or expression.

Intermediate-Level SQL Interview Questions

  • What is SQL is a self-join?

A join where a table is coupled with itself is called a self-join. Row comparisons within the same table are done using it.

  • What is the syntax to insert a row into a table in SQL?

To insert a row into a table in SQL, use the following syntax:

INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...);
  • What is a correlated subquery in SQL?

A correlated subquery is a subquery that uses values from the outer query in its WHERE clause. It’s used to filter data based on values from another table.

  • What is a common table expression (CTE) in SQL?

A common table expression (CTE) is a temporary named result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. It’s used to simplify complex queries and improve performance.

  • What is a recursive CTE in SQL?

A recursive CTE is a CTE that references itself. It’s used to traverse hierarchical data structures, such as organisational charts or file systems.

  • What is a pivot table in SQL?

A pivot table is a way to summarise data by grouping it into categories and displaying the results in a matrix. It’s used to analyse large amounts of data and make it more accessible.

  • What is the syntax to update a row in a table in SQL?

To update a row in a table in SQL, use the following syntax:

UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition;
  • What is a stored procedure vs a function in SQL?

A stored procedure is a pre-written block of code that can be called by other programs or scripts. It’s used to simplify complex tasks and improve performance. A function is a pre-written block of code that returns a single value and can be used in SQL statements just like a regular column or expression.

  • What is a cursor in SQL?

A cursor is a database object that enables traversal over the rows of a result set. It’s used to iterate over a result set and perform operations on each row.

  • What is a user-defined function in SQL?

A user-defined function is a function that is created by the user and stored in the database. It’s used to perform custom calculations or transformations on data.

  • You need to find the number of orders placed by each customer in a table that contains order data. How can you do this in SQL?

You can use the GROUP BY clause and the COUNT function to group orders by customer and count the number of orders:

SELECT customer_id, COUNT(*) FROM order_data GROUP BY customer_id;
  • What is a correlated join in SQL?

A correlated join is a join where the join condition refers to values from both tables. It’s used to filter data based on values from another table.

  • What is a group by clause in SQL?

A group by clause is used to group rows based on one or more columns. It’s used with aggregate functions, such as COUNT, SUM, AVG, MIN, and MAX, to summarise data.

  • What is a having clause in SQL?

A having clause is used to filter groups based on a condition. It’s used with group by clauses to filter data based on aggregate functions.

  • What is meant by materialised view?

A precomputed result set that is kept in a database is known as a materialised view. It is used to boost performance by cutting down on the time needed to process complicated queries.

  • What is outer join?

All the rows from one table and only the matching rows from another table are combined in an outer join. Data that might not match in another table is retrieved using it.

  • What does SQL’s clustered index mean?

A particular kind of database index called a clustered index sorts and saves the table’s data rows according to their key values. By enabling the database to discover the pertinent data rows in a table more rapidly, it helps enhance query performance.

  • What is a cursor in SQL?

A cursor is a database object that allows you to traverse the result set of a query one row at a time. It’s used to perform complex data manipulations and implement iterative processing.

  • What is a user-defined function in SQL?

A user-defined function is a database object that allows you to define your own custom functions in SQL. It’s used to simplify complex calculations and improve code reusability.

  • What is the syntax to delete a row from a table in SQL?

To delete a row from a table in SQL, use the following syntax:

DELETE FROM table_name WHERE condition;

Advanced Level SQL Interview Questions

  • You have a table with customer data that includes their names and email addresses. You need to find the email address of all customers whose name starts with “J”. How can you do this in SQL?

You can use the LIKE keyword and the % wildcard character in the WHERE clause to match all names starting with “J”:

SELECT email_address FROM customer_data WHERE name LIKE 'J%';
  • What exactly is SQL query optimization?

The practice of enhancing the efficiency of SQL queries by examining and restructuring the database tables, indexes, and query execution plan is known as query optimization.

  • How may a SQL query be optimised?

The following best practices can help you improve a SQL query:

  1. Indexes can be used to improve query performance.
  2. Avoid using SELECT * and only choose the columns that are required.
  3. To examine the query execution plan, use the EXPLAIN statement.
  4. To reduce storage and processing overhead, use appropriate data types for columns.
  • What is the difference between clustered and non-clustered indexes in SQL?

A clustered index in SQL is a type of index that sorts and stores the data rows in a table based on the index key. It is also called a table-based index because it physically reorganises the data in the table.

A non-clustered index in SQL is a type of index that does not physically reorganise the data in the table. Instead, it creates a separate data structure that points to the location of the data rows.

  • How can you select the minimum and maximum values from a column in a table in SQL?

You can select the minimum and maximum values from a column in a table in SQL using the MIN and MAX functions:

SELECT MIN(column_name), MAX(column_name) FROM table_name;
  • How can you improve the performance of a SQL query that joins multiple tables?

You can improve the performance of a SQL query that joins multiple tables by following these best practices:

  1. Use appropriate indexes on the columns used in the JOIN condition.
  2. Use INNER JOIN instead of OUTER JOIN if possible.
  3. Use SELECT statements to limit the number of columns returned by the query.
  4. Use subqueries or temporary tables to filter the data before performing the JOIN operation.
  5. Use UNION instead of JOIN if the tables have different column structures.
  • What is the syntax to create a primary key in SQL?

To create a primary key in SQL, use the following syntax:

ALTER TABLE table_name ADD CONSTRAINT constraint_name PRIMARY KEY (column1, column2, ...);
  • What distinguishes OLTP from OLAP?

OLTP and OLAP are two types of systems used for managing and processing data. 

OLTP systems are designed to handle transactional workloads, such as placing orders, making payments, and updating records, while OLAP systems are designed to handle analytical workloads, such as complex queries and generating reports based on historical data. 

OLTP databases have a normalised structure, while OLAP databases have a denormalized structure to optimise performance.

  • What is the syntax to alter a table in SQL?

To alter a table in SQL, use the following syntax:

ALTER TABLE table_name ADD column datatype;
  • What is a subquery in SQL?

A subquery in SQL is a query that is embedded within another query. It is used to retrieve data that will be used by the main query as a filter condition or to calculate a derived value.

  • How can you delete duplicate rows from a table in SQL?

You can delete duplicate rows from a table in SQL using the DISTINCT keyword and the DELETE statement:

DELETE FROM table_name WHERE column_name NOT IN (SELECT DISTINCT column_name FROM table_name);
  • What is a subquery vs a join in SQL?

A subquery is a query that is embedded within another query. It’s used to retrieve data that will be used as part of a larger query. A join is a way to combine data from two or more tables based on a common column.

  • You have a table with employee information such as names and salaries. You must identify all employees whose salaries are higher than the average. How does this work in SQL?

The AVG function can be used to calculate the average salary, and the WHERE clause can be used to filter employees whose salaries are higher than the average:

SELECT name FROM employee_data WHERE salary > (SELECT AVG(salary) FROM employee_data);
  • You have two tables, one with customer data and another with order data. You need to find the total amount of money spent by each customer. How can you do this in SQL?

You can use the JOIN keyword to join the two tables on the customer ID and the GROUP BY clause and SUM function to calculate the total amount spent by each customer:

SELECT customer_data.name, SUM(order_data.amount) FROM customer_data JOIN order_data ON customer_data.id = order_data.customer_id GROUP BY customer_data.name;
  • What exactly is a view in SQL? Provide an example.

A view is a virtual table that is formed as a result of a SQL query. It can be used to simplify difficult searches, limit access to sensitive data, or give a streamlined version of a table. As an example:

CREATE VIEW sales_data AS
SELECT customer_name, order_date, product_name, price, quantity, price * quantity AS total
FROM orders
INNER JOIN products
ON orders.product_id = products.id;
  • How can you create an index in SQL?

You can create an index in SQL using the CREATE INDEX statement followed by the name of the index, the table name, and the column name(s) to be indexed.

CREATE INDEX index_name ON table_name (column1, column2);

This will create an index named index_name on the columns column1 and column2 of the table table_name.

  • What does “multi-version concurrency control” mean to you?

Database management systems employ the Multi-Version Concurrency Control (MVCC) technique to permit concurrent access to the same data by several transactions without interfering with one another. 

  • What is RDBMS?

RDBMS is an abbreviation for Relational Database Management System. It is a piece of software used to manage and store data in relational databases. A relational database is a database type that organises data into one or more tables, each with its unique identifier known as a primary key.

  • What is an inconsistent dependency?

An inconsistent dependency is a situation where two or more requirements or conditions cannot be simultaneously satisfied.

  • What is Collation? 

Collation refers to the rules that determine how character data is sorted and compared in a database. It includes rules for character set comparison, case sensitivity, accent sensitivity, and character width.

Tips to Ace the SQL Interview on the First Attempt 

Preparing for a SQL interview can seem daunting, but with the right approach, you can increase your chances of acing it on the first attempt. 

Here are some tips to help you:

Review the Basics

Start by reviewing the basic SQL syntax and commands, such as SELECT, FROM, WHERE, GROUP BY, HAVING, and ORDER BY. Practice writing simple queries to get comfortable with the syntax.

Practice Coding Challenges

Practice coding challenges on websites such as LeetCode, HackerRank, or SQLZoo. These challenges will help you build your skills and gain experience with different types of queries and scenarios.

Understand Database Design Principles

Understand the basic principles of database design, including normalisation and relationships between tables. Be prepared to explain your understanding of these concepts and how you would apply them in a practical scenario.

Study Advanced Topics

Review more advanced topics, such as subqueries, joins, window functions, and stored procedures. Be prepared to explain how you would use these features in specific scenarios.

Prepare for Non-technical Questions

In addition to technical questions, be prepared to answer non-technical questions about your experience, strengths, weaknesses, and communication skills. Be ready to give specific examples of your experience with SQL and how you have used it in your previous work.

Stay Calm and Confident

During the interview, stay calm and confident. If you don’t know the answer to a question, be honest and explain how you would go about finding the answer.

By following these tips and practising regularly, you can increase your chances of acing the SQL interview on your first attempt.

Common Mistakes to Avoid During an Interview

When preparing for a SQL interview, there are several common mistakes that you should try to avoid:

Not Practising Enough

One of the biggest mistakes you can make is not practising enough before the interview. Make sure you practise writing SQL queries on a regular basis and take advantage of online resources and coding challenges.

Focusing Only on Syntax

While it’s important to know SQL syntax, focusing solely on syntax and not understanding the underlying principles of database design or the business context of the data can hinder your ability to answer more complex questions.

Not Asking Questions

It’s important to ask clarifying questions during the interview to fully understand the requirements of the problem you are trying to solve. Don’t be afraid to ask questions if you need more information.

Not Testing Your Code

Be sure to test your SQL queries thoroughly to make sure they produce the correct results. Don’t assume that your code is correct without testing it.

Being Too Verbose

While it’s important to explain your thought process and how you arrived at a solution, being too verbose can lead to losing the interviewer’s attention or confusing them with unnecessary details.

Not Being Familiar with the Company’s Tech Stack

Be sure to research the company and its tech stack before the interview. This will help you tailor your answers to their specific needs and demonstrate your interest in the role.

Conclusion

In conclusion, SQL is a powerful language used in database management, and being proficient in it can greatly enhance your career prospects. Preparing for a SQL interview requires a combination of technical knowledge and practical experience, as well as an understanding of database design principles and the ability to communicate effectively. It is also important to practise and checkout SQL server interview questions for better prep.

By reviewing the basics, practising coding challenges, and studying advanced topics, you can increase your chances of acing the interview. 

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