Top 50 SQLite Interview Questions for 2024

In this interview preparation blog post, we’ll explore some of the most common SQLite interview questions that can help you showcase your knowledge and skills on SQLite effectively. From understanding its fundamental concepts to tackling more advanced queries and optimization techniques, this guide aims to provide a comprehensive resource for your interview preparation.

SQLite is a powerful, embedded database engine used widely for its simplicity, reliability, and minimal configuration. As one of the most popular database systems, especially in mobile applications, web browsers, and embedded systems, it has become an essential skill for developers to master. Whether you're a seasoned developer or just starting your journey in database management, preparing for an SQLite interview can be crucial for landing your next big opportunity.

Top 50 SQLite Interview Questions

In the upcoming section, you will explore the best 50 SQLite questions that one can probably encounter during an SQLite interview.

1. What is SQLite?

SQLite is a lightweight, serverless, and self-contained relational database management system (RDBMS). Unlike other databases that require a server to run, SQLite is embedded into the application, meaning it runs within the same process as the application itself. This makes it ideal for small to medium-sized applications, including mobile apps, desktop applications, and small web applications.

2. Explain the Main Features of SQLite.

• Lightweight: SQLite is a minimalistic database engine, with a small footprint and low resource usage.
• Self-Contained: All data is stored in a single file, which makes it easy to manage and move.
• Serverless: SQLite does not require a separate server process, so there is no need for configuration or management of a database server.
• Transactional: SQLite supports ACID (Atomicity, Consistency, Isolation, Durability) transactions, ensuring reliable data operations.
• Zero Configuration: SQLite does not require setup or administration, which simplifies deployment.
• Cross-Platform: Databases created by SQLite can be used across different operating systems without modification.
• SQL Support: It supports most of the SQL standards, allowing for complex queries and operations.

3. What are the Advantages of Using SQLite?

• Easy to Use: No complex setup or configuration is required.
• Portable: The entire database is stored in a single file, making it easy to move or distribute.
• Reliable: Supports ACID transactions, which ensure data integrity.
• Fast: Performs well for read-heavy operations and moderate write operations.
• Low Maintenance: No need for a database administrator; it’s self-contained and requires minimal upkeep.
• Free: SQLite is open-source and free to use, even for commercial purposes.

4. What are the Limitations of SQLite?

• Concurrency: SQLite handles multiple readers well but can become slow with multiple simultaneous writers because it locks the entire database during write operations.
• Size: It is not ideal for very large databases (more than a few terabytes).
• Access Control: Lacks advanced access control and user management features found in other RDBMSs.
• Limited Server-Side Features: No built-in support for server-side procedures or triggers.
• Not Suitable for High-Traffic Applications: Better suited for applications with moderate traffic.

5. What is a Database in SQLite?

In SQLite, a database is a single file that stores all the data, including tables, indexes, triggers, and views. The database file can be easily transferred, backed up, or copied as needed.

6. How to Create a New SQLite Database?

To create a new SQLite database, you use the SQLite command-line interface (CLI) with the sqlite3 command followed by the desired database name. For example:

sqlite3 mydatabase.db

This command creates a new database file named mydatabase.db.

7. What is a Table in SQLite?

A table in SQLite is a structured set of data organized into rows and columns. Each table is designed to hold information about a specific topic, such as employees or products. Tables are fundamental components of a database where data is stored and managed.

8. How to Create a New Table in SQLite?

To create a new table in SQLite, you use the CREATE TABLE statement. For example:

CREATE TABLE Employees (
    EmployeeID INTEGER PRIMARY KEY,
    Name TEXT NOT NULL,
    Age INTEGER,
    Position TEXT,
    Salary REAL
);

This command creates a table named Employees with columns for EmployeeID, Name, Age, Position, and Salary.

9. What Data Types are Supported by SQLite?

SQLite uses a dynamic type system, which means you can store any type of data in any column. However, it does have affinity types, which are:

• NULL: Represents a missing value.
• INTEGER: A whole number.
• REAL: A floating-point number.
• TEXT: A text string.
• BLOB: A binary large object, used to store binary data.

10. Explain the Concept of NULL in SQLite.

In SQLite, NULL represents a missing or undefined value. It is different from zero, an empty string, or any other value. NULL is used to indicate the absence of a value in a column. Operations with NULL values generally result in NULL.

11. How to Insert Data into a Table in SQLite?

To insert data into a table, you use the INSERT INTO statement. For example:

INSERT INTO Employees (Name, Age, Position, Salary) VALUES ('John Doe', 30, 'Software Engineer', 75000);

This command updates the salary of the employee with EmployeeID 1 to 80,000.

12. How to Delete Data From a Table in SQLite?

To delete data from a table, you use the DELETE statement. For example:

DELETE FROM Employees WHERE EmployeeID = 1;

This command deletes the row from the Employees table where the EmployeeID is 1.

13. What is the SQLite Command to List All Tables in a Database?

To list all tables in an SQLite database, you use the .tables command in the SQLite command-line interface (CLI):

.tables

This command displays a list of all tables in the current database.

14. How to Retrieve Data From a Table in SQLite?

To retrieve data from a table, you use the SELECT statement. For example:

SELECT * FROM Employees;

This command retrieves all columns and rows from the Employees table. You can also specify specific columns or add conditions to filter the results.

15. How to Update Data in a Table in SQLite?

To update data in a table, you use the UPDATE statement. For example:

UPDATE Employees SET Salary = 80000 WHERE EmployeeID = 1;

This command updates the salary of the employee with EmployeeID 1 to 80,000.

16. What are SQLite Transactions?

SQLite transactions are a way to group multiple database operations into a single unit of work. A transaction ensures that either all operations within the transaction are completed successfully or none of them are. This helps maintain the integrity of the database. Transactions in SQLite are started with the BEGIN command, changes are made with INSERT, UPDATE, or DELETE commands, and the transaction is completed with the COMMIT command. If an error occurs, the transaction can be rolled back with the ROLLBACK command.

17. Explain the Concept of ACID Properties in SQLite.

ACID stands for Atomicity, Consistency, Isolation, and Durability. These are the key properties that ensure reliable database transactions:

• Atomicity: Ensures that all operations within a transaction are completed; if one operation fails, the entire transaction fails and the database remains unchanged.
• Consistency: Ensures that a transaction brings the database from one valid state to another, maintaining database rules.
• Isolation: Ensures that transactions are executed independently of each other; intermediate states of a transaction are invisible to other transactions.
• Durability: Ensures that once a transaction is committed, it will remain so, even in the event of a system failure.

18. How to Create an Index in SQLite?

An index in SQLite is created to improve the speed of data retrieval. You can create an index using the CREATE INDEX statement. For example:

CREATE INDEX idx_employee_name ON Employees (Name);

This command creates an index named idx_employee_name on the Name column of the Employees table.

19. What is a Primary Key in SQLite?

A primary key is a column or a set of columns that uniquely identifies each row in a table. It ensures that no two rows have the same primary key value and that each primary key value is unique and not NULL. In SQLite, you can define a primary key when creating a table, like this:

CREATE TABLE Employees (
    EmployeeID INTEGER PRIMARY KEY,
    Name TEXT NOT NULL,
    Age INTEGER
);

Here, EmployeeID is the primary key of the Employees table.

20. What is a Foreign Key in SQLite?

A foreign key is a column or a set of columns in one table that refers to the primary key of another table. It establishes a relationship between two tables and helps maintain referential integrity. For example:

CREATE TABLE Orders (
    OrderID INTEGER PRIMARY KEY,
    EmployeeID INTEGER,
    FOREIGN KEY (EmployeeID) REFERENCES Employees (EmployeeID)
);

Here, EmployeeID in the Orders table is a foreign key that references EmployeeID in the Employees table.

21. How to Enforce Foreign Key Constraints In SQLite?

To enforce foreign key constraints in SQLite, you need to enable foreign key support by executing the following command:

PRAGMA foreign_keys = ON;

This command ensures that all foreign key constraints are enforced. Without this, SQLite will not check foreign key constraints by default.

22. Explain the Concept of Triggers in SQLite.

Triggers are special stored procedures that are automatically executed or fired when certain events occur in the database, such as INSERT, UPDATE, or DELETE. Triggers help enforce business rules, validate input data, and maintain audit trails. They are defined to respond to specific changes in the database.

23. How to Create a Trigger in SQLite?

You create a trigger using the CREATE TRIGGER statement. For example:

CREATE TRIGGER update_employee_salary
AFTER UPDATE ON Employees
FOR EACH ROW
BEGIN
    UPDATE Payroll SET Salary = NEW.Salary WHERE EmployeeID = NEW.EmployeeID;
END;

This trigger updates the Salary in the Payroll table whenever the Salary of an employee is updated in the Employees table.

24. What are Views in SQLite?

Views in SQLite are virtual tables created by a SELECT query. They do not store data themselves but present data from one or more tables in a specific format. Views help simplify complex queries and provide a way to present data in different ways without duplicating data.

25. How to Create a View in SQLite?

You create a view using the CREATE VIEW statement. For example:

CREATE VIEW EmployeeDetails AS
SELECT EmployeeID, Name, Position, Salary
FROM Employees;

This command creates a view named EmployeeDetails that shows specific columns from the Employees table.

26. Explain the Concept of SQLite Schemas.

A schema in SQLite defines the structure of the database, including tables, views, indexes, and triggers. It provides a blueprint for how data is organized and how relationships are maintained within the database. The schema includes the SQL commands that create and maintain these structures.

27. How to Use the ATTACH Command in SQLite?

The ATTACH command is used to attach another database file to the current database connection. This allows you to execute queries across multiple databases. For example:

ATTACH 'other_database.db' AS otherDB;

This command attaches the other_database.db file and makes its contents accessible under the otherDB schema.

28. What are the Different Join Types Supported By SQLite?

SQLite supports several types of joins, including:

• INNER JOIN: Returns rows when there is a match in both tables.
• LEFT JOIN (or LEFT OUTER JOIN): Returns all rows from the left table and matched rows from the right table. If no match, NULL is returned for columns from the right table.
• RIGHT JOIN (or RIGHT OUTER JOIN): Returns all rows from the right table and matched rows from the left table. If no match, NULL is returned for columns from the left table. (Not directly supported, can be achieved with LEFT JOIN and table order swapping)
• FULL JOIN (or FULL OUTER JOIN): Returns rows when there is a match in one of the tables. (Not directly supported, can be achieved with a combination of LEFT JOIN, RIGHT JOIN, and UNION)
• CROSS JOIN: Returns the Cartesian product of both tables.

29. How to Perform a Join Operation in SQLite?

To perform a join operation, you use the JOIN keyword in a SELECT statement. For example, an INNER JOIN between Employees and Orders:

SELECT Employees.Name, Orders.OrderID
FROM Employees
INNER JOIN Orders ON Employees.EmployeeID = Orders.EmployeeID;

This query retrieves the names of employees and their corresponding order IDs where there is a match between EmployeeID in both tables.

30. Explain the Concept of Virtual Tables in SQLite.

Virtual tables in SQLite are tables whose data is not stored in the database file itself but is instead provided by an external source, such as a custom application code or an extension. Virtual tables allow you to integrate non-database data into SQLite queries as if it were part of the database. This is useful for integrating data from other sources or implementing custom storage engines.

31. What is the SQLite PRAGMA Command and How is It Used?

The SQLite PRAGMA command is a special SQL statement used to query or change the operational parameters of the SQLite library. PRAGMAs allow you to control various database behaviors and retrieve information about the database. They are useful for tasks such as setting foreign key constraints, configuring the journal mode, or retrieving the database schema. For example:

PRAGMA foreign_keys = ON;

This command enables foreign key constraints in the database.

32. Explain the Concept of SQLite WAL (Write-Ahead Logging).

Write-Ahead Logging (WAL) is a journaling mode in SQLite that provides better performance and concurrency. In WAL mode, instead of overwriting the original database file, changes are first written to a separate WAL file. The changes are later copied from the WAL file back to the original database file during a checkpoint operation. This allows readers to access the original database file while writers continue making changes in the WAL file, thus improving concurrency. To enable WAL mode, use:

PRAGMA journal_mode = WAL;

33. How to Optimize Queries in SQLite?

Optimizing queries in SQLite involves several strategies to improve performance:

• Indexing: Create indexes on columns that are frequently used in WHERE clauses, JOIN conditions, and sorting operations.

CREATE INDEX idx_name ON Employees (Name);

• Using EXPLAIN: Use the EXPLAIN keyword to analyze the query execution plan and identify performance bottlenecks.

EXPLAIN QUERY PLAN SELECT * FROM Employees WHERE Name = 'John Doe';

• Avoiding SELECT : Specify only the columns you need instead of using SELECT *.
• Limiting Result Set: Use the LIMIT clause to reduce the number of rows returned.

SELECT * FROM Employees LIMIT 10;

• Optimizing Joins: Ensure proper indexing of columns used in join conditions and use the appropriate join type.

34. What is the Use of the ANALYZE Command in SQLite?

The ANALYZE command in SQLite is used to gather statistics about the distribution of data in tables and indexes. These statistics help the query planner to make better decisions about how to execute queries efficiently. By analyzing the database, you can improve query performance. To use the ANALYZE command, simply execute:

ANALYZE;

This command analyzes the entire database. You can also analyze a specific table or index:

ANALYZE Employees;
ANALYZE idx_name;

35. How to Handle Concurrency in SQLite?

SQLite handles concurrency using locks to control access to the database. The database can be in one of several locking states: unlocked, shared, reserved, pending, or exclusive. SQLite uses a simple locking mechanism to manage concurrent access:

• Readers: Multiple readers can access the database simultaneously.
  
• Writers: Only one writer can modify the database at a time. When a writer starts a transaction, it acquires an exclusive lock, preventing other writers and readers from accessing the database.

To ensure proper concurrency, design your application to minimize the duration of write operations and use transactions to group related operations.

36. What are the Different Types of Locks in SQLite?

SQLite uses the following types of locks to manage access to the database:

• UNLOCKED: No locks are held.
• SHARED: Multiple readers can acquire a shared lock, allowing concurrent read access.
• RESERVED: A writer acquires a reserved lock before making changes, allowing existing readers to finish but preventing new readers.
• PENDING: A writer acquires a pending lock before acquiring an exclusive lock, indicating that it intends to write.
• EXCLUSIVE: Only one writer can hold an exclusive lock, preventing all other read and write access.

37. Explain the Concept of SQLite Full-Text Search (FTS).

Full-Text Search (FTS) in SQLite is a feature that allows you to efficiently search large text-based data. FTS provides specialized indexing and querying capabilities for full-text search, making it suitable for applications like document indexing, search engines, and content management systems. SQLite offers FTS modules such as FTS3, FTS4, and FTS5 to enable full-text search functionality.

38. How to Perform Full-Text Search in SQLite?

To perform full-text search in SQLite, you need to create a virtual table using one of the FTS modules and then use the MATCH operator to query the table. For example:

CREATE VIRTUAL TABLE documents USING fts5(content);
INSERT INTO documents (content) VALUES ('This is a sample document.');
INSERT INTO documents (content) VALUES ('Another document with some text.');

SELECT * FROM documents WHERE content MATCH 'sample';

This example creates an FTS5 virtual table, inserts documents, and searches for the word "sample" in the documents.

39. What are the Best Practices For Securing an SQLite Database?

To secure an SQLite database, consider the following best practices:

• Use Encryption: Use SQLite Encryption Extension (SEE) or third-party encryption libraries to encrypt the database file.
• Access Control: Implement proper access controls to restrict who can read or write to the database file.
• Secure Storage: Store the database file in a secure location with appropriate file system permissions.
• Sanitize Inputs: Always sanitize user inputs to prevent SQL injection attacks.
• Backup: Regularly backup the database to prevent data loss.

40. How to Backup And Restore an SQLite Database?

To backup an SQLite database, you can use the .backup command in the SQLite command-line interface or use the sqlite3_backup_* API functions in your application code. For example, using the command-line interface:

sqlite3 source.db ".backup backup.db"

This command creates a backup of source.db to backup.db.

To restore an SQLite database, you can use the .restore command in the SQLite command-line interface:

sqlite3 new.db ".restore backup.db"

This command restores the contents of backup.db to new.db.

SQLite Query Based Interview Questions

Departments Table

CREATE TABLE Departments (
    DepartmentID INTEGER PRIMARY KEY AUTOINCREMENT,
    DepartmentName TEXT NOT NULL
);

INSERT INTO Departments (DepartmentName) VALUES
('Engineering'), ('Design'), ('Management');

Output:

Employees Table:

CREATE TABLE Employee (
    EmployeeID INTEGER PRIMARY KEY AUTOINCREMENT,
    Name TEXT NOT NULL,
    Age INTEGER,
    Position TEXT,
    Salary REAL,
    DepartmentID INTEGER,
    FOREIGN KEY (DepartmentID) REFERENCES Departments (DepartmentID)
);

INSERT INTO Employee (Name, Age, Position, Salary, DepartmentID) VALUES
('John Doe', 28, 'Software Engineer', 80000, 1),
('Jane Smith', 34, 'Project Manager', 95000, 1),
('Emily Johnson', 41, 'CTO', 150000, 3),
('Michael Brown', 29, 'Software Engineer', 85000, 1),
('Sarah Davis', 26, 'UI/UX Designer', 70000, 2);

Output:

Projects Table:

CREATE TABLE Projects (
    ProjectID INTEGER PRIMARY KEY AUTOINCREMENT,
    ProjectName TEXT NOT NULL,
    Budget REAL,
    DepartmentID INTEGER,
    FOREIGN KEY (DepartmentID) REFERENCES Departments (DepartmentID)
);

INSERT INTO Projects (ProjectName, Budget, DepartmentID) VALUES
('Project Alpha', 100000, 1),
('Project Beta', 200000, 2),
('Project Gamma', 150000, 3);

Output:

Tasks Table:

CREATE TABLE Tasks (
    TaskID INTEGER PRIMARY KEY AUTOINCREMENT,
    TaskName TEXT NOT NULL,
    ProjectID INTEGER,
    AssignedTo INTEGER,
    Status TEXT,
    FOREIGN KEY (ProjectID) REFERENCES Projects (ProjectID),
    FOREIGN KEY (AssignedTo) REFERENCES Employees (EmployeeID)
);


INSERT INTO Tasks (TaskName, ProjectID, AssignedTo, Status) VALUES
('Design Database', 1, 1, 'Completed'),
('Develop API', 1, 1, 'In Progress'),
('Create UI', 2, 5, 'Not Started'),
('Project Planning', 3, 2, 'Completed'),
('Market Analysis', 3, 3, 'In Progress');

Output:

41. List all employees who are working on projects managed by their own department.

Query:

SELECT e.Name
FROM Employee e
JOIN Tasks t ON e.EmployeeID = t.AssignedTo
JOIN Projects p ON t.ProjectID = p.ProjectID
WHERE e.DepartmentID = p.DepartmentID;

Output:

Explanation: This query joins the Employee, Tasks, and Projects tables to find employees who are assigned to tasks on projects that belong to their own department.

42. Find the total budget of projects managed by each department and order by the total budget in descending order.

Query:

SELECT d.DepartmentName, SUM(p.Budget) as TotalBudget
FROM Departments d
JOIN Projects p ON d.DepartmentID = p.DepartmentID
GROUP BY d.DepartmentName
ORDER BY TotalBudget DESC;

Output:

Explanation: This query calculates the total budget of projects for each department by joining the Departments and Projects tables, grouping by DepartmentName, and ordering the result by TotalBudget in descending order.

43. Retrieve the names of employees who are not assigned to any task.

Query:

SELECT Name
FROM Employee e
LEFT JOIN Tasks t ON e.EmployeeID = t.AssignedTo
WHERE t.TaskID IS NULL;

Output:

Explanation: This query uses a left join to find employees who are not assigned to any task by checking for NULL values in the TaskID field of the Tasks table.

44. List the names of employees and the number of tasks they are assigned to, ordered by the number of tasks in descending order.

Query:

SELECT e.Name, COUNT(t.TaskID) as TaskCount
FROM Employee e
LEFT JOIN Tasks t ON e.EmployeeID = t.AssignedTo
GROUP BY e.Name
ORDER BY TaskCount DESC;

Output:

Explanation: This query counts the number of tasks assigned to each employee by joining the Employee and Tasks tables, grouping by employee name, and ordering by TaskCount in descending order.

45. Find the employees who have a salary higher than the average salary of their respective department.

Query:

SELECT e.Name, e.Salary, d.DepartmentName
FROM Employee e
JOIN Departments d ON e.DepartmentID = d.DepartmentID
WHERE e.Salary > (SELECT AVG(e2.Salary)
                  FROM Employee e2
                  WHERE e2.DepartmentID = e.DepartmentID);

Output:

Explanation: This query finds employees whose salary is higher than the average salary of their respective department by comparing each employee's salary to the subquery result that calculates the average salary for their department.

46. Retrieve the names of employees who are assigned to all projects managed by their department.

Query:

SELECT e.Name
FROM Employee e
JOIN Tasks t ON e.EmployeeID = t.AssignedTo
JOIN Projects p ON t.ProjectID = p.ProjectID
WHERE e.DepartmentID = p.DepartmentID
GROUP BY e.Name
HAVING COUNT(DISTINCT t.ProjectID) = (SELECT COUNT(*)
                                      FROM Projects p2
                                      WHERE p2.DepartmentID = e.DepartmentID);

Output:

Explanation: This query identifies employees who are assigned to every project in their department by grouping by employee name and ensuring the count of distinct projects they are assigned to matches the total number of projects in their department.

47. List the projects and the names of employees who have not been assigned any task in that project.

Query:

SELECT p.ProjectName, e.Name
FROM Projects p
CROSS JOIN Employee e
LEFT JOIN Tasks t ON p.ProjectID = t.ProjectID AND e.EmployeeID = t.AssignedTo
WHERE t.TaskID IS NULL;

Output:

Explanation: This query lists projects and employees who are not assigned to any tasks in that project by using a cross join to consider all possible combinations and then filtering out the assigned ones with a left join and a NULL check.

48. Find the department that has the highest average salary among its employees.

Query:

SELECT d.DepartmentName
FROM Departments d
JOIN Employee e ON d.DepartmentID = e.DepartmentID
GROUP BY d.DepartmentName
ORDER BY AVG(e.Salary) DESC
LIMIT 1;

Output:

Explanation: This query finds the department with the highest average salary by joining the Departments and Employee tables, grouping by department name, ordering by average salary in descending order, and limiting the result to the top one.

49. List the names of employees who have never worked on a completed task.

Query:

SELECT e.Name
FROM Employee e
WHERE NOT EXISTS (SELECT 1
                  FROM Tasks t
                  WHERE t.AssignedTo = e.EmployeeID
                  AND t.Status = 'Completed');

Output:

Explanation: This query lists employees who have never worked on a completed task by checking for the absence of any task with a status of 'Completed' for each employee using the NOT EXISTS clause.

50. Find the project with the highest number of tasks assigned.

Query:

SELECT p.ProjectName, COUNT(t.TaskID) as TaskCount
FROM Projects p
JOIN Tasks t ON p.ProjectID = t.ProjectID
GROUP BY p.ProjectName
ORDER BY TaskCount DESC
LIMIT 1;

Output:

Explanation: This query finds the project with the highest number of tasks by joining the Projects and Tasks tables, grouping by project name, counting the number of tasks, ordering by task count in descending order, and limiting the result to the top one.

Conclusion

Understanding SQLite is important for developers, especially those working with mobile apps, websites, and embedded systems. Preparing for an SQLite interview means knowing both the basics and more advanced features. This guide has covered many common interview questions to help you get ready. By practicing these questions and answers, you'll be better prepared to show your skills and knowledge to employers.