25 Most Common SQL Interview Questions

SQL, or Structured Query Language, is a powerful tool for managing and manipulating data within relational database management systems (RDBMS). For anyone navigating the vast landscape of SQL interview questions, a solid understanding of SQL fundamentals is crucial.

Let’s explore common SQL interview questions and explore into the answers.

What is SQL?

SQL stands for Structured Query Language. It is a domain-specific language used to manage and manipulate relational databases. Approved as a standard by the American National Standards Institute (ANSI) in 1986, SQL serves as a universal language for interacting with various database management systems.

What Can SQL Do for You?

SQL offers a comprehensive set of capabilities for working with databases:

Query Execution: SQL can run queries against a database.
Data Retrieval: It can retrieve information from a database.
Record Creation: SQL can create new records in a database.
Data Updating: It can update existing data in a database.
Record Deletion: SQL can delete records from a database.
Database Creation: SQL can create entirely new databases.
Table Creation: It can create new tables within a database.
Stored Procedures: SQL enables the creation of stored procedures.
Views: SQL can generate views in a database.
Permissions Management: It allows the establishment of permissions on tables, procedures, and views.

Basic SQL Interview Questions

1Q: How do you distinguish between SQL and MySQL?

A: SQL is a standard language for managing relational databases. MySQL, on the other hand, is a specific type of relational database management system (RDBMS) that utilizes SQL as its query language. In essence, SQL is the language, while MySQL is one of the databases that employ SQL.

2Q: What are the various SQL subsets?

A: SQL subsets are categorized into:

Data Definition Language (DDL): Involves creating, altering, and deleting items in the database.
Data Manipulation Language (DML): Deals with accessing and altering data, including operations like insert, update, delete, and select.
Data Control Language (DCL): Manages database access permissions through actions like granting and revoking.

3Q: What is a Database Management System (DBMS), and what are its types?

A: A DBMS is software that interacts with users, applications, and the database itself to capture and analyze data. Two main types are:

Relational Database Management System (RDBMS): Organizes data into tables with relationships (e.g., MySQL).
Non-Relational Database Management System: Lacks tables, tuples, or attributes (e.g., MongoDB).

4Q: In SQL, how do you define a table and a field?

A: A table is a logically organized collection of data in rows and columns. A field refers to the columns in a table, representing different attributes. For example, a table might have fields such as Student ID, Student Name, and Student Marks.

5Q: How do you define joins in SQL?

A: A join clause combines rows from two or more tables based on a common column. There are various types:

Inner Join: Retrieves rows with matching values in both tables.
Full Join: Returns all records when there is a match in any table.
Right Join: Returns all rows from the right table and matching rows from the left table.
Left Join: Returns all rows from the left table and matching rows from the right table.

6Q: What is the difference between SQL data types CHAR and VARCHAR2?

A: Both CHAR and VARCHAR2 are used for character strings. The key difference is that CHAR is for fixed-length strings, while VARCHAR2 is for variable-length strings. For example, CHAR(10) can only hold 10 characters, while VARCHAR2(10) can store any length.

7Q: What are constraints in SQL?

A: Constraints in SQL are used to set limits on the type of data that can be stored in a table. Examples include:

UNIQUE: Ensures uniqueness of values in a column.
NOT NULL: Requires a column to have a value, disallowing NULL.
FOREIGN KEY: Enforces referential integrity.
DEFAULT: Sets a default value for a column.
CHECK: Verifies a condition.
PRIMARY KEY: Identifies a unique record in a table.

8Q: What is a foreign key?

A: A foreign key is a field in a database table that links to the primary key in another table. It establishes referential integrity, ensuring data consistency between related tables.

9Q: What is “data integrity”?

A: Data integrity refers to the accuracy and consistency of data stored in a database. It involves maintaining the quality and reliability of data and enforcing integrity constraints to preserve data accuracy during input and modification.

10Q: What is the difference between a clustered and a non-clustered index?

A: The key distinctions between clustered and non-clustered indexes are:

Clustered Index: Alters the physical order of rows in a table based on the indexed column. There can be only one clustered index per table.
Non-Clustered Index: Does not change the physical order of the table. It creates a separate object pointing to the original table rows. Multiple non-clustered indexes can exist in a table.

11Q: How would you write an SQL query to show the current date?

A: The current date can be obtained using a built-in SQL function, such as `GETDATE()`.

12Q: What is “query optimization”?

A: Query optimization involves identifying the most efficient plan for executing a query with the lowest projected cost. It aims to enhance query performance and reduce the complexity of time and space.

13Q: What is “denormalization”?

A: Denormalization is a technique in which redundancy is introduced into a table to improve overall database performance. It involves merging data from multiple tables into a single table, enhancing query speed by reducing the need for joins.

14Q: What are the differences between entities and relationships?

A: Entities represent real-world people, places, or things stored in a database table. Relationships denote connections between entities or tables, reflecting shared characteristics or dependencies.

15Q: What is an index?

A: An index is a performance optimization mechanism used to retrieve records from a table quickly. By creating an entry for each value, data retrieval becomes faster.

16Q: Describe the various types of indexes in SQL.

A: SQL indexes come in three types:

Unique Index: Ensures uniqueness, preventing duplicate values.
Clustered Index: Reorders the table physically based on key values.
Non-Clustered Index: Does not change the physical order of data but maintains a logical order.

17Q: What is normalization, and what are its benefits?

A: Normalization is the process of structuring data in a database to minimize redundancy and improve data integrity. Benefits include:

Improved database management
Compact database size
Efficient data access
Enhanced query flexibility
Quick data retrieval
Simplified security implementation

18Q: Describe the various forms of normalization.

A: The primary normalization forms include:

First Normal Form (1NF): Eliminates repeating groups between rows.
Second Normal Form (2NF): Ensures every non-key column relies on the primary key.
Third Normal Form (3NF): Ensures each non-key column depends only on the primary key.

19Q: In a database, what is the ACID property?

A: ACID stands for Atomicity, Consistency, Isolation, and Durability. It ensures reliable data transactions:

Atomicity: A transaction is completed or failed as a whole.
Consistency: Data adheres to all validation standards.
Isolation: Ensures concurrency control.
Durability: Committed transactions persist despite external factors.

20Q: What is a “Trigger” in SQL?

A: A trigger in SQL is a stored procedure configured to execute automatically before or after data changes. It allows the execution of a batch of code upon specific events like insert, update, or delete operations.

21Q: What are the different types of SQL operators?

A: SQL operators include logical operators, arithmetic operators, and comparison operators.

22Q: Do NULL values have the same meaning as zero or a blank space?

A: No, NULL values represent an unavailable, unknown, or not applicable value. They are distinct from zero (a numerical value) or a blank space (a character value).

23Q: What is the difference between a natural join and a cross join?

A: A natural join relies on columns with the same name and data types in both tables. A cross join, on the other hand, creates the Cartesian product of two tables, resulting in all possible combinations.

24Q: What is a subquery in SQL?

A: A subquery is a query embedded within another query. The inner query is processed first, and its result is passed to the outer query. Subqueries can be used in various SQL statements, including SELECT, UPDATE, and others.

25Q: What are the various forms of subqueries?

A: Subqueries come in two forms:

Correlated Subquery: References another table and column from the outer query.
Non-Correlated Subquery: Functions independently, with its output replacing the main query results.

Mastering these SQL interview questions provides a strong foundation for navigating the dynamic world of database management systems. Whether you’re a seasoned SQL professional or a newcomer to the language, these questions cover key concepts essential for success in SQL-related interviews.

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