Top 50 Interview Questions and Answers for SQL

SQL (Structured Query Language) is one of the most versatile languages used in industry today. It is a powerful database programming language used for creating and managing data held in relational databases. One can pursue online SQL certification courses to enhance their skills in this language.

SQL skills are highly sought after by employers. Thus, it is important to brush up on your interview questions for SQL before heading into an interview. In this article, we have covered some of the top interview questions and answers for SQL to help you learn the SQL basics for an interview.

1. What is SQL?

This is one of the top SQL interview questions to know. SQL is a standard computer language for relational database management and data manipulation. It is used to query, insert, update, and delete data in databases. It is also used to create and modify database structures (tables, views, and indices).

SQL is a declarative language, which means that the programmer specifies what actions should be taken, but not how those actions are to be performed. It is also a procedural language, which means that it can be used to write programs that specify the steps to be taken to perform an action.

2. What are the different types of SQL statements?

There are several different types of SQL statements, including:

• Data Manipulation Language (DML): These statements are used to retrieve, insert, update, and delete data from a database.

• Data Definition Language (DDL): These statements are used to create and modify database objects such as tables, views, and indexes.

• Transaction Control Statements: These statements are used to manage transactions within a database.

• Session Control Statements: These statements are used to control the session in which a user is connected to a database.

3. What is the difference between a primary key and a foreign key?

This is one of the top SQL interview questions. A primary key is a unique identifier for a specific row in a table, while a foreign key is a field in one table that refers to the primary key in another table.

The primary key ensures that each row in the table is uniquely identifiable, while the foreign key ensures that data in one table is linked to data in another table.

4. What is the purpose of an index in a database?

An index is used to improve the performance of database queries by allowing the database to quickly locate the data that is being searched for. An index is essentially a data structure that maps the values in a specific column to the location of the corresponding rows in the table.

By using an index, the database can avoid scanning the entire table and instead quickly retrieve the relevant data.

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5. How would you select all customers from a database who live in California?

To select all customers from a database who live in California, it is essential to use an SQL SELECT statement with a WHERE clause that specifies the condition for the query. For example:

SELECT * FROM customers WHERE state = 'California';

This statement would retrieve all rows from the "customers" table where the value in the "state" column is equal to "California".

6. What is a join, and how would you use it to retrieve data from multiple tables?

This is one of the important interview questions and answers for SQL. A join is used to combine data from two or more tables into a single result set. To use a join to retrieve data from multiple tables, you would use an SQL SELECT statement that includes the table names and the join condition. For example:

SELECT customers.name, orders.order_date

FROM customers

INNER JOIN orders

ON customers.customer_id = orders.customer_id;

This statement would retrieve the name of each customer and the order date for all orders placed by that customer. The INNER JOIN clause specifies that only rows that have a matching customer ID in both the "customers" and "orders" tables should be included in the result set.

7. How would you insert a new row into a database table?

To insert a new row into a database table, you would use an SQL INSERT statement that specifies the table name and the values to be inserted into each column. For example:

INSERT INTO customers (name, address, city, state, zip)

VALUES ('John Smith', '123 Main St', 'San Francisco', 'California', '94103');

This statement would insert a new row into the "customers" table with the specified values in each column.

8. How would you update information in an existing row of a database table?

To update information in an existing row of a database table, you would use an SQL UPDATE statement that specifies the table name, the columns to be updated, and the new values for those columns. For example:

UPDATE customers

SET state = 'New York'

WHERE customer_id = 123;

This statement would update the "state" column for the row with a customer ID of 123 to "New York".

9. How would you delete a row from a database table?

To delete a row from a database table, you would use an SQL DELETE statement that specifies the table name and the condition for the rows to be deleted. For example:

DELETE FROM customers

WHERE customer_id = 123;

This statement would delete the row from the "customers" table where the customer ID is equal to 123.

10. What is the difference between a LEFT JOIN and an INNER JOIN in SQL?

A LEFT JOIN returns all rows from the left table and matching rows from the right table. If there is no matching row in the right table, NULL values are returned for the right table columns. In contrast, an INNER JOIN only returns rows that have matching values in both the left and right tables.

This means that if there is no matching row in the right table, the row from the left table is not included in the result set. It is important to understand the differences between these two types of joins in order to effectively retrieve the desired data from multiple tables in a database.

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11. What is an SQL constraint, and why is it important in a database?

A SQL constraint is a rule that defines how data in a database table should be handled. Constraints are used to enforce data integrity and ensure that data is accurate and consistent. Common types of constraints include primary keys, foreign keys, unique constraints, and check constraints.

12. Explain the concept of normalisation in database design.

Normalisation is the process of organising data in a database to reduce redundancy and improve data integrity. It involves breaking down large tables into smaller, related tables and creating relationships between them.

Normalisation helps prevent data anomalies and improves data consistency. This is one of the SQL interview questions for freshers to practice.

13. What is an SQL subquery, and how does it differ from a JOIN?

An SQL subquery is a query nested within another query. It is used to retrieve data that will be used as part of the main query's condition. Subqueries can return a single value, a list of values, or even a result set.

Unlike a JOIN, which combines data from multiple tables into a single result set, a subquery is used within a single query to filter or manipulate data.

14. What is the purpose of the GROUP BY clause in SQL, and when is it used?

The GROUP BY clause in SQL is used to group rows that have the same values in specified columns into summary rows. It is typically used with aggregate functions like COUNT, SUM, or AVG, to perform calculations on groups of data. GROUP BY is commonly used in reporting and data analysis to summarise and aggregate data.

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15. Explain the concept of an SQL view and its advantages.

An SQL view is a virtual table that is derived from one or more base tables. Views are used to simplify complex queries, provide a level of abstraction, and restrict access to certain columns or rows of data.

They offer the advantage of simplifying query writing, enhancing security, and reducing redundancy in SQL code. This is one of the SQL interview questions for experienced professionals.

16. What is the purpose of the HAVING clause in SQL, and when is it used?

The HAVING clause is used in conjunction with the GROUP BY clause to filter rows in the result set based on the result of an aggregate function. It is applied after data has been grouped, allowing you to filter groups of data based on specified conditions.

HAVING is used to filter grouped data, whereas the WHERE clause filters individual rows.

17. Explain SQL injection and how it can be prevented.

SQL injection is a security vulnerability that occurs when an attacker inserts malicious SQL code into the input fields of a web application. This can lead to unauthorised access to a database or data manipulation.

To prevent SQL injection, use parameterised queries, prepared statements, and input validation to ensure that user input is treated as data rather than executable SQL code.

18. What is a self-join in SQL, and when is it used?

A self-join is a type of SQL join where a table is joined with itself. It is used when a table contains a hierarchical structure or when you need to compare rows within the same table.

For example, you might use a self-join to find all employees who report to the same manager in an employee hierarchy. This is one of the important SQL questions for your interview.

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19. Explain the concept of ACID properties in database transactions.

ACID stands for Atomicity, Consistency, Isolation, and Durability. These properties ensure that database transactions are reliable and maintain data integrity:

Atomicity: Transactions are treated as a single unit, and they are either completed in their entirety or not at all.

Consistency: Transactions bring the database from one consistent state to another, following all defined rules and constraints.

Isolation: Transactions are executed independently and do not interfere with each other.

Durability: Once a transaction is committed, its changes are permanent and will survive system failures.

20. What is the difference between UNION and UNION ALL in SQL, and when would you use each?

UNION and UNION ALL are used to combine the results of two or more SELECT statements into a single result set. The key difference is that UNION removes duplicate rows from the result set, while UNION ALL includes all rows, including duplicates.

When someone wants to eliminate duplicate rows can use UNION and to retain duplicates, use UNION ALL can be used.

21. What is the purpose of the SQL CASE statement, and how is it used?

The SQL CASE statement is used to perform conditional logic within a query. It allows you to create conditional expressions based on specified conditions and return different values depending on the outcome.

The CASE statement can be used in SELECT, WHERE, and ORDER BY clauses, among others. This is one of the SQL basic interview questions to prepare for.

22. Explain the concept of a SQL stored procedure and its advantages.

A SQL stored procedure is a precompiled and reusable block of SQL code that can be executed on demand. They are used to encapsulate business logic, enhance security, improve performance, and simplify maintenance.

Stored procedures are particularly useful when the same set of SQL statements needs to be executed multiple times.

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23. What is the purpose of the SQL TRUNCATE statement, and how does it differ from DELETE?

The SQL TRUNCATE statement is used to remove all rows from a table quickly. It is a non-logged operation and is generally faster than DELETE, which removes rows one at a time and generates log entries. TRUNCATE also resets identity columns to their seed value, whereas DELETE does not.

24. Explain the concept of SQL indexes and their role in query optimization.

SQL indexes are data structures that improve the speed of data retrieval operations on database tables. They work by creating a copy of selected columns and their values, allowing the database engine to locate rows more efficiently.

Indexes are crucial for query optimization, as they reduce the need for full table scans, resulting in faster query execution. This is one of the SQL interview questions and answers for freshers and professionals alike.

25. What is the purpose of the SQL ORDER BY clause, and how is it used?

The SQL ORDER BY clause is used to sort the result set of a query in ascending or descending order based on one or more columns. It is often used to present data in a specific order for reporting or presentation purposes. You can specify the sorting order for each column in the ORDER BY clause.

26. Explain the concept of SQL transactions and their importance.

SQL transactions are a sequence of one or more SQL statements that are treated as a single, atomic unit of work. Transactions ensure that a series of database operations either complete successfully (commit) or leave the database in its original state (rollback) in case of an error. They are vital for maintaining data consistency and integrity.

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27. What is the difference between a database and a DBMS (Database Management System)?

A database is a structured collection of data, while a DBMS (Database Management System) is software used to manage, store, and manipulate data within a database.

The DBMS provides an interface for users and applications to interact with the database, including functions for data storage, retrieval, and security. This is one of the SQL basic questions to prepare for your interview.

28. Explain the concept of SQL data types and give examples of different data types.

SQL data types define the type of data that can be stored in a column of a database table. Common data types include:

• INTEGER: Used for whole numbers.
• VARCHAR or CHAR: Used for character strings.
• DATE or TIMESTAMP: Used for date and time values.
• BOOLEAN: Used for true or false values.
• BLOB: Used for binary large objects, such as images or documents.

29. Explain the concept of database normalisation forms (1NF, 2NF, 3NF).

Database normalisation is a process to eliminate data redundancy and improve data integrity. The common normalisation forms include:

• First Normal Form (1NF): Ensures that each column contains atomic (indivisible) values.

• Second Normal Form (2NF): Builds on 1NF and eliminates partial dependencies by moving non-key attributes to separate tables.

• Third Normal Form (3NF): Builds on 2NF and eliminates transitive dependencies by removing attributes that depend on other non-key attributes.

30. What is an SQL cursor, and when is it used?

An SQL cursor is a database object that is used to traverse and manipulate data row by row within a result set. Cursors are typically used within stored procedures or functions to process individual rows of data sequentially.

They are helpful when you need to perform row-level operations. This type of SQL queries for interviews are important to practice.

31. Explain the concept of SQL primary key constraints and their significance.

A primary key constraint is used to uniquely identify each row in a table. It ensures that the values in the specified column or columns are unique and not null. Primary keys are crucial for data integrity and enforcing data uniqueness in a table. They are used as references in foreign key relationships.

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32. What is the purpose of SQL views and how are they different from tables?

SQL views are virtual tables that do not store data themselves but are defined by SQL queries. They provide a way to present data from one or more tables in a structured manner.

Views offer advantages such as simplifying complex queries, enhancing security, and hiding the underlying table structure. Unlike tables, views do not store physical data.

33. Explain the concept of SQL foreign key constraints and their role in maintaining data integrity.

This is one of the common SQL technical interview questions. A foreign key constraint is used to establish a relationship between two tables in a database. It enforces referential integrity by ensuring that values in a specific column (the foreign key) of one table correspond to values in another table's primary key. Foreign keys help maintain data consistency and prevent orphaned records.

34. What is an SQL trigger, and how does it differ from a stored procedure?

An SQL trigger is a database object that automatically executes a predefined set of actions in response to specific events or conditions, such as data modification. A stored procedure, on the other hand, is a reusable block of SQL code that can be executed on demand.

While both triggers and stored procedures contain SQL code, triggers are event-driven and executed automatically, while stored procedures are called explicitly by a user or application.

35. What is the purpose of the SQL UNION operator, and how does it work?

The SQL UNION operator is used to combine the results of two or more SELECT statements into a single result set. It removes duplicate rows by default. To use UNION, the SELECT statements must have the same number of columns, and the corresponding columns must have compatible data types.

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36. Explain the concept of SQL indexing and its impact on database performance.

SQL indexing involves creating data structures (indexes) that improve the speed of data retrieval operations. Indexes work by mapping the values in specific columns to the location of corresponding rows in a table.

They reduce the need for full table scans, leading to faster query execution. However, indexes consume storage space and require maintenance. This type of SQL questions and answers should be prepared for an interview.

37. What is SQL normalisation, and why is it essential in database design?

SQL normalisation is a process that organises data in a database to eliminate redundancy and improve data integrity. It involves breaking down large tables into smaller, related tables and establishing relationships between them. Normalisation is essential because it prevents data anomalies, ensures data consistency, and simplifies database maintenance.

38. What is an SQL stored procedure, and what are its advantages?

An SQL stored procedure is a precompiled block of SQL code that can be executed on demand. It allows for the encapsulation of business logic within the database, enhances security by controlling data access, improves performance by reducing network traffic, and simplifies maintenance by centralising code. Stored procedures are especially useful for repetitive or complex database operations.

39. Explain the concept of SQL transaction isolation levels and their significance.

This is one of the basic SQL queries interview questions. SQL transaction isolation levels determine how transactions interact with each other in a multi-user database environment. Common isolation levels include READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.

Each level offers a different balance between concurrency and data consistency. Understanding isolation levels is crucial for managing data integrity in multi-user databases.

40. What is SQL injection, and how can it be prevented in web applications?

SQL injection is a security vulnerability that occurs when an attacker manipulates user input to inject malicious SQL code into an application's database queries. This can lead to unauthorised access to the database or data manipulation.

To prevent SQL injection, developers should use parameterised queries or prepared statements and validate and sanitise user input to ensure that it is treated as data, not executable code.

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41. Explain the concept of SQL cursors and their use cases.

SQL cursors are database objects used to traverse and manipulate data row by row within a result set. They are often employed in stored procedures or functions when individual rows need to be processed sequentially.

Cursors are useful when performing row-level operations, such as calculations, updates, or deletions, on large datasets.

42. What is the purpose of the SQL COMMIT and ROLLBACK statements?

The SQL COMMIT statement is used to permanently save the changes made during a transaction. It marks the end of a successful transaction and makes the changes permanent in the database.

In contrast, the SQL ROLLBACK statement is used to undo the changes made during a transaction and return the database to its original state. It is used when a transaction encounters an error or is intentionally rolled back. This is one of the common SQL interview questions.

43. Explain the concept of SQL views and their advantages in database management.

SQL views are virtual tables that do not store data themselves but are defined by SQL queries. They provide a way to present data from one or more tables in a structured manner.

Views offer advantages such as simplifying complex queries, enhancing security by limiting access to specific columns, and hiding the underlying table structure. They are particularly useful for presenting data to users and applications.

44. What is the purpose of SQL subqueries, and how are they different from joins?

SQL subqueries are queries nested within another query. They are used to retrieve data that is used as part of the main query's condition or result. Subqueries can return a single value, a list of values, or even a result set.

Unlike joins, which combine data from multiple tables into a single result set, subqueries are used within a single query to filter or manipulate data.

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45. Explain the concept of SQL indexing and its impact on query performance.

SQL indexing involves creating data structures (indexes) that improve the speed of data retrieval operations. Indexes work by mapping the values in specific columns to the location of corresponding rows in a table.

SQL indexing reduces the need for full table scans, resulting in faster query execution. However, indexes consume storage space and require maintenance to remain effective. This is one of the important interview questions for SQL.

46. What is the difference between SQL INNER JOIN and LEFT JOIN, and when would you use each?

SQL INNER JOIN and LEFT JOIN are types of joins used to combine data from multiple tables. The main difference is in the results they produce:

INNER JOIN returns only rows that have matching values in both tables. Rows without matches are excluded.

LEFT JOIN returns all rows from the left table and matching rows from the right table. If there is no match in the right table, NULL values are returned.

47. What is the purpose of SQL functions, and what are some common built-in functions?

SQL functions are pre-defined operations that perform a specific task on data in a database. They are used for calculations, data manipulation, and transformation. Common built-in functions include:

SUM(): It calculates the sum of values in a column.

AVG(): It calculates the average of values in a column.

MAX(): It retrieves the maximum value from a column.

MIN(): It retrieves the minimum value from a column.

COUNT(): It counts the number of rows in a result set.

48. What is an SQL index, and how does it improve query performance?

An SQL index is a data structure that enhances the speed of data retrieval operations on database tables. It works by creating a copy of selected columns and their values, allowing the database engine to locate rows more efficiently.

An SQL Index minimises the need for full table scans, leading to faster query execution. This is one of the top SQL interview questions.

49. Explain the concept of SQL data manipulation commands.

SQL data manipulation commands are used to manipulate data in a database. Some common data manipulation commands include:

INSERT: Adds new rows to a table.

UPDATE: Modifies existing data in a table.

DELETE: Removes rows from a table.

MERGE: Combines data from multiple sources into a target table.

50. What is the purpose of SQL constraints, and what are some common types of constraints?

SQL constraints are rules that define how data in a database table should be handled. They enforce data integrity and ensure that data is accurate and consistent. Common types of constraints include:

PRIMARY KEY: Ensures uniqueness and identifies a unique record.

FOREIGN KEY: Establishes relationships between tables.

UNIQUE: Ensures that values in a column are unique.

CHECK: Enforces a condition on data values.

NOT NULL: Requires a column to have a non-null value.

For instance, a PRIMARY KEY constraint on the "customer_id" column ensures each customer record is unique and identifiable. This is one of the frequently asked interview questions and answers for SQL.

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Conclusion

When gearing up for SQL interviews, thorough preparation is key. Start by understanding SQL basics and syntax through online resources. Be ready to demonstrate your SQL knowledge and discuss your experience with various commands and data querying. Prepare examples of your SQL work to showcase your skills.

Prepare for questions on database design principles and optimisation, and practice real-life examples of good practices. Additionally, have insightful questions ready for the interviewer to display your interest and research about the company.

Most importantly, practice extensively, simulating real interview scenarios with a partner or by writing out answers to sample questions. This comprehensive approach ensures readiness for any SQL interview, reinforcing the importance of practice and familiarity with core concepts.