SQLite Interview Questions and Answers

1. What is SQLite?

   • Answer: SQLite is a lightweight, embedded, serverless, and self-contained SQL database engine. It's a single file database, meaning the entire database is stored in a single file on the disk. It's commonly used in mobile apps, embedded systems, and other applications where a full-blown database server isn't necessary or practical.

2. What are the advantages of using SQLite?

   • Answer: Advantages include its simplicity, ease of use, zero-configuration requirements, cross-platform compatibility, small footprint, and excellent performance for many applications. It's also easy to embed directly into applications.

3. What are the disadvantages of using SQLite?

   • Answer: Disadvantages include potential concurrency limitations (although significantly improved in recent versions), lack of advanced features found in larger database systems (like replication or sophisticated transactions), and potential vulnerability to data corruption if the database file is not properly handled.

4. How does SQLite handle transactions?

   • Answer: SQLite supports ACID properties (Atomicity, Consistency, Isolation, Durability) through transactions managed using `BEGIN TRANSACTION`, `COMMIT`, and `ROLLBACK` statements. These ensure data integrity even during concurrent access or failures.

5. Explain the difference between `INTEGER`, `REAL`, `TEXT`, and `BLOB` data types in SQLite.

   • Answer: `INTEGER` stores integers, `REAL` stores floating-point numbers, `TEXT` stores text strings, and `BLOB` stores binary data (images, etc.). SQLite is relatively flexible with type checking, automatically converting between types in some cases.

6. What is an SQL injection vulnerability, and how can it be prevented in SQLite?

   • Answer: SQL injection allows attackers to inject malicious SQL code into database queries, potentially compromising data. Prevention in SQLite involves using parameterized queries or prepared statements, which prevent the injection of arbitrary SQL code.

7. How do you create a new table in SQLite?

   • Answer: Use the `CREATE TABLE` statement. For example: `CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT, email TEXT);`

8. How do you insert data into a SQLite table?

   • Answer: Use the `INSERT INTO` statement. For example: `INSERT INTO users (name, email) VALUES ('John Doe', 'john.doe@example.com');`

9. How do you query data from a SQLite table?

   • Answer: Use the `SELECT` statement. For example: `SELECT * FROM users;` or `SELECT name, email FROM users WHERE id = 1;`

10. How do you update data in a SQLite table?

    • Answer: Use the `UPDATE` statement. For example: `UPDATE users SET email = 'john.updated@example.com' WHERE id = 1;`

11. How do you delete data from a SQLite table?

    • Answer: Use the `DELETE FROM` statement. For example: `DELETE FROM users WHERE id = 1;`

12. What is a PRIMARY KEY constraint?

    • Answer: A PRIMARY KEY uniquely identifies each row in a table. It cannot contain NULL values and must be unique.

13. What is a FOREIGN KEY constraint?

    • Answer: A FOREIGN KEY constraint establishes a link between two tables, creating a referential integrity relationship. It ensures that values in the foreign key column exist in the corresponding primary key column of another table.

14. What are indexes in SQLite, and why are they useful?

    • Answer: Indexes are data structures that improve the speed of data retrieval. They work by creating a sorted lookup table for a particular column or set of columns, allowing SQLite to quickly locate rows matching specific search criteria.

15. How do you create an index in SQLite?

    • Answer: Use the `CREATE INDEX` statement. For example: `CREATE INDEX idx_user_name ON users (name);`

16. What is the difference between `INNER JOIN`, `LEFT JOIN`, and `RIGHT JOIN`?

    • Answer: `INNER JOIN` returns rows only when there is a match in both tables. `LEFT JOIN` returns all rows from the left table, and matching rows from the right table (NULLs if no match). `RIGHT JOIN` is the opposite, returning all rows from the right table.

17. Explain the use of `WHERE` clause in SQL queries.

    • Answer: The `WHERE` clause filters the rows returned by a `SELECT` statement, based on specified conditions.

18. Explain the use of `ORDER BY` clause in SQL queries.

    • Answer: The `ORDER BY` clause sorts the rows returned by a `SELECT` statement, based on specified columns in ascending or descending order.

19. Explain the use of `LIMIT` clause in SQL queries.

    • Answer: The `LIMIT` clause restricts the number of rows returned by a `SELECT` statement.

20. What is a `GROUP BY` clause, and how is it used?

    • Answer: The `GROUP BY` clause groups rows with the same values in specified columns into summary rows, often used with aggregate functions like `COUNT`, `SUM`, `AVG`, etc.

21. What are aggregate functions in SQLite? Give examples.

    • Answer: Aggregate functions perform calculations on a set of values and return a single value. Examples include `COUNT()`, `SUM()`, `AVG()`, `MAX()`, `MIN()`, etc.

22. What is a subquery, and how is it used?

    • Answer: A subquery is a query nested inside another query. It's used to filter data, retrieve data, or perform calculations that are used in the main query.

23. How do you handle NULL values in SQLite?

    • Answer: Use `IS NULL` or `IS NOT NULL` in the `WHERE` clause to check for NULL values. Aggregate functions usually ignore NULLs; use `COALESCE` to provide a default value for NULLs.

24. What are transactions in SQLite, and why are they important?

    • Answer: Transactions are a sequence of database operations treated as a single unit of work. They guarantee atomicity and data consistency, ensuring that either all operations in a transaction succeed or none do.

25. How do you use `VACUUM` in SQLite?

    • Answer: `VACUUM` reclaims unused space in the database file, reducing its size and improving performance. It should be used cautiously, as it can be time-consuming.

26. Explain the concept of a database trigger in SQLite.

    • Answer: A trigger is a procedural code that automatically executes in response to certain events on a particular table, such as INSERT, UPDATE, or DELETE.

27. How do you create a view in SQLite?

    • Answer: Use the `CREATE VIEW` statement to define a virtual table based on the result-set of an SQL statement.

28. What are the different ways to connect to a SQLite database from a programming language?

    • Answer: Many programming languages have libraries for connecting to SQLite. Examples include the `sqlite3` module in Python, JDBC drivers in Java, and various libraries in other languages like C++, PHP, etc.

29. How do you manage concurrency in SQLite?

    • Answer: SQLite uses file-level locking mechanisms to manage concurrency. While it supports multiple readers, writers need exclusive access. Proper transaction management and careful consideration of locking are essential.

30. What are the different storage engines available in SQLite?

    • Answer: SQLite primarily uses a single storage engine, which is a B-tree based approach. It doesn't offer the choice of different storage engines like some other database systems.

31. How do you handle large datasets in SQLite?

    • Answer: Optimizations include creating appropriate indexes, using `WHERE` clauses effectively, optimizing queries, using `LIMIT` and `OFFSET` for pagination, and potentially considering alternative database systems for extremely large datasets.

32. How do you perform full-text search in SQLite?

    • Answer: Use the FTS (Full-Text Search) extension. This provides features like stemming, ranking, and wildcard searching.

33. What are temporary tables in SQLite, and how are they used?

    • Answer: Temporary tables exist only for the duration of a connection. They are useful for intermediate results or temporary data storage during a session.

34. What is the `ATTACH` command in SQLite?

    • Answer: `ATTACH` allows you to access multiple databases within a single database connection.

35. How do you export data from a SQLite database?

    • Answer: Use the `.export` command in the SQLite command-line shell or use programming language libraries to fetch data and write it to a file (CSV, JSON, etc.).

36. How do you import data into a SQLite database?

    • Answer: Use the `.import` command in the SQLite command-line shell or use programming language libraries to read data from a file and execute `INSERT` statements.

37. What are the different ways to backup a SQLite database?

    • Answer: Simply copying the database file is the simplest backup. More sophisticated methods involve using scripting to create regular backups or using database replication techniques (although not native to SQLite).

38. How do you handle errors in SQLite?

    • Answer: Use error handling mechanisms provided by the programming language's database library. Check for error codes returned by database functions.

39. What is the role of the `pragma` command in SQLite?

    • Answer: `PRAGMA` commands are used to configure aspects of the database, such as setting various database properties (e.g., `PRAGMA synchronous = OFF;`).

40. Explain the concept of database schema in SQLite.

    • Answer: The database schema defines the structure of the database, including tables, columns, data types, constraints, indexes, and relationships between tables.

41. How do you improve the performance of SQLite queries?

    • Answer: Use appropriate indexes, optimize queries (avoid using `SELECT *`), use `EXPLAIN QUERY PLAN` to analyze query execution, and ensure the database file is not fragmented (use `VACUUM`).

42. What is the difference between `AUTOINCREMENT` and `ROWID` in SQLite?

    • Answer: `ROWID` is a system-assigned unique identifier for each row. `AUTOINCREMENT` ensures that new rows automatically receive unique, sequentially increasing integer values in a specified column.

43. How do you use `CASE` statements in SQLite?

    • Answer: `CASE` statements allow conditional logic within SQL queries, providing different results based on different conditions.

44. How do you use `LIKE` operator in SQLite for pattern matching?

    • Answer: `LIKE` is used for pattern matching in strings using wildcards like `%` (any sequence of characters) and `_` (single character).

45. What is the purpose of the `ANALYZE` command in SQLite?

    • Answer: `ANALYZE` updates the database statistics used by the query planner to improve query optimization.

46. How do you work with dates and times in SQLite?

    • Answer: Use the `TEXT` data type to store dates and times in a specific format (e.g., YYYY-MM-DD HH:MM:SS) and use date and time functions for manipulation and comparison.

47. What are user-defined functions (UDFs) in SQLite?

    • Answer: UDFs allow extending SQLite functionality by creating custom functions written in a programming language like C or Python.

48. How do you enable the FTS5 extension in SQLite?

    • Answer: It depends on how you're using SQLite. If compiling from source, you need to enable it during compilation. If using a pre-built version, you might need to load it dynamically (if supported).

49. How do you handle large text fields in SQLite efficiently?

    • Answer: Consider using appropriate indexing strategies, avoid unnecessary retrieval of large text fields, and potentially use techniques like storing large text externally and referencing it in the database.

50. What is the WAL (Write-Ahead Logging) mode in SQLite?

    • Answer: WAL mode improves concurrency by separating write operations from database file updates, allowing readers to access the database while writers are active.

51. How do you use transactions to maintain data integrity in SQLite?

    • Answer: Use `BEGIN TRANSACTION`, `COMMIT`, and `ROLLBACK` to group database operations into atomic units, ensuring that either all operations succeed or none do, maintaining data consistency.

52. What is the `integrity_check` pragma in SQLite?

    • Answer: `PRAGMA integrity_check` verifies the integrity of the database by checking for constraint violations. It's a useful tool for detecting data corruption.

53. How do you troubleshoot performance issues in SQLite?

    • Answer: Use `EXPLAIN QUERY PLAN` to analyze query execution, check for missing indexes, optimize queries, and consider upgrading hardware or using more efficient data structures if necessary.

54. How do you secure a SQLite database?

    • Answer: Secure file system permissions are crucial. Prevent direct access to the database file by unauthorized users. Use parameterized queries to prevent SQL injection vulnerabilities.

55. What is the difference between SQLite and other relational database management systems (RDBMS)?

    • Answer: Key differences include SQLite's serverless nature (no separate server process), its lightweight design, embedded nature, and limited features compared to larger RDBMS like MySQL, PostgreSQL, or Oracle. SQLite is optimized for embedding into applications, while others are designed for larger-scale, client-server deployments.

56. How does SQLite handle data types differently from other RDBMS?

    • Answer: SQLite's type system is more flexible and less strict than others; it often performs automatic type conversions. Other RDBMS enforce stricter type checking.

57. How would you choose between SQLite and a client-server database system for a project?

    • Answer: Choose SQLite for embedded systems, mobile apps, or smaller applications where a lightweight database is sufficient and where managing a separate database server is undesirable. Choose a client-server RDBMS for larger applications with high concurrency requirements, demanding performance, or sophisticated features like replication and advanced transaction management.

58. What are some common use cases for SQLite?

    • Answer: Common use cases include mobile apps, embedded systems, desktop applications, web applications (sometimes), and applications where a simple and self-contained database is preferred.

59. What tools can be used to manage and administer SQLite databases?

    • Answer: The SQLite command-line shell is a fundamental tool. GUI tools like DB Browser for SQLite provide a user-friendly interface for managing databases.

60. How does SQLite handle data corruption?

    • Answer: It's important to use proper techniques to prevent corruption, including using transactions, file system backups, and regularly checking database integrity using `PRAGMA integrity_check`. If corruption occurs, data recovery may be challenging, possibly requiring specialized tools.

61. What are some best practices for using SQLite in an application?

    • Answer: Use transactions, use parameterized queries to prevent SQL injection, create appropriate indexes, optimize queries, handle errors gracefully, and back up the database regularly.

62. Explain the concept of a journal in SQLite.

    • Answer: The journal is a file used to record database changes during transactions. It ensures atomicity and durability; if a crash occurs, the journal is used to restore the database to a consistent state.

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