30+ Most Asked SQL Interview Questions and Answers

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Best SQL Interview Questions and Answers

Delve into the world of data management with Instaily Academy’s exceptional SQL training in Kolkata, led by industry veterans. Our trainers, recognised for their expertise and teaching excellence, have meticulously crafted this collection of interview questions and answers, drawing upon their extensive experience as developers, designers, and debuggers in top companies.

Q1. What is SQL?

Ans: SQL, or Structured Query Language, is a specialized programming language designed for managing and manipulating relational databases. It serves as a standardized means of interacting with databases, allowing users to define, retrieve, update, and manipulate data. SQL plays a pivotal role in database management, providing a powerful and efficient way to work with structured information. It’s the language of choice for tasks ranging from simple data queries to complex database transactions, making it an integral skill for anyone involved in data-driven applications and systems.

Q2. What is a Database?

Ans: A database is a systematically organized and electronically stored collection of data. It serves as a centralized repository for various types of information, enabling efficient data management and retrieval. Databases are designed to store, organize, and provide quick access to data, making them fundamental in diverse applications. They play a crucial role in facilitating structured storage, retrieval, and manipulation of information, making them an essential component in the architecture of software systems and applications.

Q3. What is DBMS?

Ans: A Database Management System (DBMS) is specialized software that facilitates the creation, organization, and management of databases. Acting as an intermediary between users and databases, DBMS provides an interface for defining, storing, retrieving, and managing data efficiently. Its primary functions include data definition (creating and modifying database structures), data manipulation (inserting, updating, and deleting data), and ensuring data integrity and security. DBMS plays a pivotal role in handling large volumes of data, enabling users and applications to interact seamlessly with databases while ensuring the accuracy and reliability of the stored information.

Q4. What is RDBMS? How is it different from DBMS?

Ans: RDBMS (Relational Database Management System) is a specific category of Database Management System that organizes data into tables with predefined relationships between them. The key distinction from a generic DBMS lies in its adherence to the relational model, enforcing the principles of integrity, consistency, and relationships among data entities.

While both DBMS and RDBMS serve the purpose of managing databases, an RDBMS, by design, introduces a structured and interconnected approach to data organization. It ensures that data is stored in a way that maintains relationships between tables, allowing for efficient querying and retrieval based on these relationships.

In summary, the primary difference lies in the organizational structure and adherence to the relational model within an RDBMS, which enhances data consistency and integrity through predefined constraints and relationships.

Q5. Does SQL support programming language features?

Ans: While SQL (Structured Query Language) is not a traditional programming language, it does incorporate certain programming language features. SQL is primarily a declarative language focused on querying and manipulating data in databases. It allows users to define, retrieve, update, and manage data without specifying the exact steps for how to achieve those tasks.

However, some relational database management systems (RDBMS) extend SQL to include procedural elements, creating a hybrid known as PL/SQL (Procedural Language/Structured Query Language). PL/SQL integrates procedural constructs such as loops, conditional statements, and exception handling into SQL, providing a more comprehensive set of tools for data manipulation.

So, while SQL itself may lack some features of traditional programming languages, the integration of procedural elements in extensions like PL/SQL allows for a broader range of functionality in certain database environments.

Q6. What is the difference between SQL and PL/SQL?

Ans: 

Q7. What is the difference between BETWEEN and IN operators in SQL?

Ans: Common comparison between the BETWEEN and IN operators in SQL:

Q8. What are Tables and Fields?

Ans: Tables: Tables are the fundamental building blocks of relational databases, including those used in SQL (Structured Query Language). They store data in rows and columns, with each row representing a single record and each column representing a specific attribute of that record. 

Fields: Fields, also known as columns, are vertical partitions within a table that store specific data attributes. They define the characteristics of the data that will be stored in each column, such as data type (e.g., text, number, date, Boolean), length, and constraints. 

Q9. What are Constraints in SQL?

Ans: Constraints in SQL are rules that are used to specify the type and format of data that can be stored in a table. Constraints help to ensure the accuracy, consistency, and reliability of the data in the table. Constraints can also prevent invalid or unwanted operations on the data, such as inserting, updating, or deleting records that violate the rules.

Q10. What is a Primary Key?

Ans: A Primary Key in SQL is a singular, unique identifier for each record in a table, ensuring data distinctiveness. It serves a pivotal role in maintaining data integrity by preventing the insertion of duplicate records and establishing referential integrity in relationships with other tables. The primary key column(s) must contain unique, non-null values and is typically indexed for efficient data retrieval. This key is crucial for relational database design, enhancing data accuracy and supporting seamless linkage between tables. In essence, it acts as a cornerstone for uniquely identifying and retrieving specific records within a database.

Q11. What is the difference between CHAR and VARCHAR2 data type in SQL?

Ans: In SQL, the distinction between CHAR and VARCHAR2 data types lies in how they handle character strings. 

CHAR is a fixed-length data type, allocating a set amount of storage regardless of the actual content length. CHAR is suitable for fixed-size data, ensuring consistency, but may lead to wasted storage. 

In contrast, VARCHAR2 is a variable-length data type, using storage only for the actual data length. VARCHAR2 is more flexible, efficient for varying-length strings, optimizing storage usage. 

When choosing between them, considerations include the nature of the data and the trade-off between fixed-size precision and dynamic storage efficiency.

Q12. What is a UNIQUE constraint?

Ans: In SQL, a UNIQUE constraint ensures that all values in a specified column or a combination of columns are unique across the entire table. It prohibits the insertion of duplicate values, contributing to data integrity. The UNIQUE constraint can be applied to a single column or a group of columns, providing flexibility in defining uniqueness criteria. This constraint is valuable for preventing the storage of identical records, enhancing the accuracy and reliability of the data within a database.

Q13. Name different types of case manipulation functions available in SQL.

Ans: In SQL, various case manipulation functions are available to modify the case of strings. Some common ones include:

UPPER(): Converts all characters in a string to uppercase.

SELECT UPPER(column_name) FROM table_name;

LOWER(): Converts all characters in a string to lowercase.

SELECT LOWER(column_name) FROM table_name;

INITCAP(): Capitalizes the first letter of each word in a string.

SELECT INITCAP(column_name) FROM table_name;

UCASE(): Equivalent to UPPER(), converts all characters in a string to uppercase.

SELECT UCASE(column_name) FROM table_name;

LCASE(): Equivalent to LOWER(), converts all characters in a string to lowercase.

SELECT LCASE(column_name) FROM table_name;

Q14. What is a Foreign Key?

Ans: A foreign key is a field or a set of fields in a table that refers to the primary key of another table. It establishes a link or relationship between the two tables based on the values of those fields. The purpose of a foreign key is to enforce referential integrity between the two tables.

Example of creating a foreign key in SQL:

CREATE TABLE Orders (

 OrderID INT PRIMARY KEY,

 CustomerID INT,

 -- Other columns

 FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)

In this example, the CustomerID column in the Orders table is a foreign key that references the CustomerID column in the Customers table.

Advanced Level SQL Interview Questions

Q15. What do you mean by data definition language?

Ans: Data Definition Language (DDL) in SQL is a subset of commands that deal with the structural elements of a database. DDL statements are responsible for defining and managing the structure of database objects such as tables, views, indexes, and schemas. Key DDL commands include CREATE (for creating objects), ALTER (for modifying object structures), and DROP (for deleting objects). DDL operations focus on defining the database’s organization and structure, enabling users to create, modify, and manage the database schema. This subset of SQL is crucial for shaping the foundation of a database and ensuring its integrity and organizational coherence.

Q16. What is a Join? List its different types.

Ans: A join is an operation that combines rows from two or more tables based on related columns in SQL. It allows for the retrieval of data from multiple tables in a single result set. The main types of joins are:

• INNER JOIN: Retrieves rows from both tables where there is a match based on the specified condition.
• LEFT JOIN (or LEFT OUTER JOIN): Retrieves all rows from the left table and matching rows from the right table. Non-matching rows in the right table contain NULL values.
• RIGHT JOIN (or RIGHT OUTER JOIN): Similar to LEFT JOIN but retrieves all rows from the right table and matching rows from the left table.
• FULL JOIN (or FULL OUTER JOIN): Retrieves all rows when there is a match in either the left or right table. Non-matching rows contain NULL values.
• CROSS JOIN: Returns the Cartesian product of both tables, combining every row from the left table with every row from the right table.

Q17. What is a Self-Join?

Ans: A Self-Join in SQL occurs when a table is joined with itself. This type of join is used when there is a need to combine rows within the same table based on related columns. In a self-join, the table is referenced with aliases to distinguish between the roles it plays in the join operation. It allows for comparisons between different rows within the same table, facilitating relationships or hierarchies within the data. Self-joins are commonly employed when dealing with hierarchical structures, such as organizational charts or network graphs, where relationships exist within the same entity or table.

Q18. What is a Cross-Join?

Ans: A Cross-Join, also known as a Cartesian Join, in SQL is an operation that returns the Cartesian product of two or more tables involved in the join. Unlike other join types that rely on specified conditions for matching, a Cross-Join combines every row from the first table with every row from the second table, resulting in a large result set. This type of join is generally used sparingly due to its potential to generate a substantial number of rows, which can lead to performance issues. Cross-Joins are explicitly specified using the CROSS JOIN keyword in SQL queries.

Q19. What do you mean by data manipulation language?

Ans: Data Manipulation Language (DML) in SQL is a subset of commands used to manipulate data stored in a database. DML operations primarily involve querying, inserting, updating, and deleting records within tables. Key DML commands include:

• SELECT: Retrieves data from one or more tables based on specified criteria.
• INSERT: Adds new records into a table.
• UPDATE: Modifies existing records in a table.
• DELETE: Removes records from a table based on specified conditions.

DML commands are essential for interacting with and modifying the content of a database, enabling users to retrieve, insert, update, and delete data as needed.

Q20. What is the difference between primary key and unique constraints?

Ans: The primary difference between a Primary Key and Unique Constraint lies in their roles and implications within a relational database.

Primary Key:

• Uniqueness: A Primary Key ensures uniqueness for each record in a table, and no two records can have the same primary key value.
• Null Values: A Primary Key column cannot contain NULL values, ensuring every record has a unique identifier.
• Table Relationship: It often serves as a reference for establishing relationships with other tables, enforcing referential integrity.
• Single per Table: Each table can have only one Primary Key.

Unique Constraint:

• Uniqueness: Like a Primary Key, a Unique Constraint ensures that values in a column or a combination of columns are unique across the entire table.
• Null Values: Unlike a Primary Key, a Unique Constraint allows NULL values in the constrained columns, allowing for unique but potentially nullable entries.
• Table Relationship: While it enforces uniqueness, a Unique Constraint is not typically used as a reference for establishing relationships with other tables.
• Multiple per Table: A table can have multiple Unique Constraints, allowing for different unique conditions on various columns.

Q21. What is an Index? Explain its different types.

Ans: An Index is a database object that enhances the speed of data retrieval operations on a table. It works by providing a quick reference to the location of rows based on the values in one or more columns. Different types of indexes include:

• Single-Column Index: Created on a single column, improving the retrieval speed of queries that involve that column.
• Composite Index (or Multi-Column Index): Created on multiple columns, optimizing queries that involve the specified columns collectively.
• Unique Index: Ensures the uniqueness of values in the indexed column(s), preventing the insertion of duplicate entries.
• Clustered Index: Dictates the physical order of data in the table, affecting the arrangement of rows based on the indexed column(s).
• Non-Clustered Index: Provides a separate structure to store index data, allowing for a different physical order than the actual data in the table.

Understanding and appropriately implementing indexes are crucial for optimizing query performance and enhancing the efficiency of database operations.

Q22. What is the difference between Clustered and Non-clustered index?

Ans: The primary difference between a Clustered Index and a Non-Clustered Index lies in how they organize and store data within a table.

Q23. What is the view in SQL?

Ans: A view in SQL is a virtual table derived from a SELECT query. It simplifies complex queries, enhances security, and provides a convenient way to present data to users without exposing the underlying table structure.

Q24. What is the ON DELETE CASCADE constraint?

Ans: The ON DELETE CASCADE constraint ensures referential integrity by automatically deleting related records in child tables when a referenced record in the parent table is deleted.

Q25. What is Data Integrity?

Ans: Data integrity ensures the accuracy, consistency, and reliability of data in a database. It is maintained through constraints like UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK constraints.

Q26. What is a Query?

Ans: A query is a request for information from a database. It typically involves SELECT statements to retrieve specific data based on defined criteria.

Q27. What is a Subquery? What are its types?

Ans: A subquery is a nested query within a main query. It can be correlated or non-correlated. Correlated subqueries reference columns from the outer query, while non-correlated subqueries can run independently.

Q28. What is Normalization?

Ans: Normalization is the process of organizing data to reduce redundancy and dependency. It involves dividing tables into smaller, related tables to minimize data duplication.

Q29. What is Denormalization?

Ans: Denormalization is the opposite of normalization. It involves combining tables to simplify queries and improve performance. It’s often used in data warehousing for analytical purposes.

Q30. Explain the WITH clause in SQL?

Ans: The WITH clause defines a common table expression (CTE) for use within a SELECT, INSERT, UPDATE, or DELETE statement. It enhances readability and simplifies complex queries.

Q31. What is the SELECT statement?

Ans: The SELECT statement retrieves data from one or more tables. It allows users to specify the columns to retrieve, filter rows, and sort results.

Q32. What are some common clauses used with the SELECT query in SQL?

Ans: Common clauses include WHERE for filtering, ORDER BY for sorting, GROUP BY for grouping, and HAVING for filtering grouped results.

Q33. What are UNION, MINUS, and INTERSECT commands?

Ans: UNION combines the results of two SELECT queries, MINUS returns unique rows from the first query but not the second, and INTERSECT returns common rows between two queries.

Q34. What are all the different attributes of indexes?

Ans: Attributes include uniqueness, efficiency, and data distribution. Indexes can be single-column or composite, unique or non-unique, and clustered or non-clustered.

Q35. What is a Cursor? How to use a Cursor?

Ans: A Cursor is a database object used to navigate and manipulate result sets row by row. It is declared, opened, fetched, and closed in SQL statements or stored procedures.

Q36. Write down various types of relationships in SQL?

Ans: Relationships include one-to-one, one-to-many, and many-to-many. They are established using primary and foreign keys to ensure referential integrity.

Q37. What is an Alias in SQL?

Ans: An Alias in SQL is a temporary name assigned to a table or column during a query. It enhances readability and is often used when dealing with complex queries involving multiple tables.