MYSQL and Mongo DB Interview

Certainly! Here's a brief explanation of each topic in layman's terms:

1. Introduction to DBMS:

   • DBMS is like a digital file cabinet where you can store, organize, and retrieve information in a structured way.

2. Primary Key in DBMS:

   • Think of a primary key as a unique ID for each row in a table. It helps identify and access specific records easily.

3. Foreign Key in DBMS:

   • A foreign key is like a reference link between two tables. It helps establish connections and relationships between data in different tables.

4. Constraints in SQL:

   • Constraints are rules that you set for your data, ensuring it meets specific conditions. It's like defining what values are allowed in certain columns.

5. All Types of SQL Commands with Example:

   • SQL commands are like instructions for your database. They tell it what to do, like adding data, modifying records, or retrieving information.

6. Difference between Delete, Drop & Truncate in SQL:

   • Delete removes specific data, Drop deletes an entire table, and Truncate removes all data from a table. Each has different levels of impact.

7. Differentiate Group by and Order by:

   • Group by is like sorting data based on common values, while Order by is arranging the data in ascending or descending order.

8. Types of Joins:

   • Joins combine data from different tables. Think of it like merging information from two spreadsheets based on a common column.

9. Nested Subquery VS Correlated Subquery:

   • Nested Subquery is like a query inside another query, and Correlated Subquery is a query that refers to values from the outer query. Both help in complex data retrieval.

10. Pattern Matching:

    • Pattern matching, like using the 'LIKE' command, helps find data based on specific patterns or characters.

11. Find 2nd highest salary:

    • It's a query to retrieve the second-highest salary from a database table.

12. SQL vs MySQL:

    • SQL is a language for managing databases, while MySQL is a specific type of database software that uses SQL.

13. Varchar vs Varchar2:

    • Both are data types in SQL. Varchar is for variable-length character strings, and Varchar2 is an improved version with additional features.

14. Triggers:

    • Triggers are like automatic actions that occur when certain events (like data changes) happen in the database.

15. ACID Properties:

    • ACID properties ensure that database transactions are reliable. It's like a set of rules to guarantee data integrity, even if something goes wrong.

16. DBMS & RDBMS:

    • DBMS: Manages databases but doesn't necessarily enforce relationships.
    • RDBMS: Organizes data into tables with enforced relationships between them.

17. Primary Key:

    • Unique ID for each row in a table; ensures data integrity.

18. Foreign Key:

    • Links tables by referencing the Primary Key in another table; establishes relationships.

19. Constraints & their types:

    • Constraints: Rules for data integrity.
    • Types: Ensure uniqueness (Unique), enforce relationships (Foreign Key), and define conditions (Check).

20. Diff types of SQL commands:

    • Instructions for databases, including adding data (INSERT), modifying (UPDATE), retrieving (SELECT), and more.

21. Diff b/w Delete, Drop & Truncate:

    • Delete: Removes specific data.
    • Drop: Deletes an entire table.
    • Truncate: Removes all data from a table.

22. Diff b/w Group by & Order by:

    • Group by: Organizes data based on common values.
    • Order by: Arranges data in ascending or descending order.

23. Types of Joins:

    • Combine data from different tables; includes Inner, Outer, Left, and Right Joins.

24. Nested subquery vs Correlated subquery:

    • Nested Subquery: Query inside another query.
    • Correlated Subquery: Refers to values from the outer query.

25. Pattern Matching:

    • Finding data based on specific patterns or characters.

26. Find 2nd or Nth highest salary:

    • Query to retrieve the second or Nth highest salary from a table.

27. SQL vs MySQL (NoSQL):

    • SQL: Language for managing databases.
    • MySQL (NoSQL): Specific database software using SQL; NoSQL databases like MongoDB use a different approach.

28. Varchar vs Varchar2:

    • Varchar: Variable-length character strings.
    • Varchar2: Improved version with additional features.

29. Triggers:

    • Automatic actions triggered by events like data changes.

30. ACID properties:

    • Rules ensuring reliable database transactions: Atomicity, Consistency, Isolation, and Durability.

Sure, let's provide the MongoDB equivalents or explanations for each topic:

1. DBMS & RDBMS in MongoDB:

   • MongoDB is a NoSQL document-oriented database, which means it doesn't strictly follow the structure of traditional RDBMS. MongoDB is often categorized as a DBMS, but not an RDBMS because it doesn't use tables with fixed schemas.

2. Primary Key in MongoDB:

   • In MongoDB, the primary key equivalent is the _id field. Each document in a collection automatically has a unique _id field.

3. Foreign Key in MongoDB:

   • MongoDB uses a different approach called embedding or referencing to represent relationships between documents. There is no concept of a foreign key in the traditional sense.

4. Constraints & their types in MongoDB:

   • MongoDB is schema-less, so constraints like primary key or unique key constraints are not explicitly enforced. Validation rules can be defined using MongoDB's schema validation features.

5. Diff types of MongoDB commands:

   • MongoDB has commands for CRUD operations: insert, find, update, and remove. These commands operate on BSON (Binary JSON) documents.

6. Diff b/w Delete, Drop & Truncate in MongoDB:

   • MongoDB doesn't have a concept of TRUNCATE. deleteOne and deleteMany are used for deletion. drop is used to drop a collection (similar to dropping a table).

7. Diff b/w Group by & Order by in MongoDB:

   • MongoDB's aggregate pipeline is used for grouping similar to GROUP BY. Sorting is done using sort (equivalent to ORDER BY).

8. Types of Joins in MongoDB:

   • MongoDB doesn't support traditional joins. Instead, you model data based on the query patterns your application needs, often using embedding or referencing.

9. Nested subquery vs Correlated subquery in MongoDB:

   • MongoDB doesn't have a concept of subqueries in the traditional sense. You structure your data based on how it will be queried.

10. Pattern Matching in MongoDB:

    • MongoDB uses regular expressions for pattern matching in queries.

11. Find 2nd or Nth highest value in MongoDB:

    • This involves using sort and skip in MongoDB to skip the first N-1 documents.

12. MongoDB (NoSQL) vs SQL (Relational databases):

    • MongoDB is a NoSQL database, emphasizing flexibility and scalability. It stores data in flexible, JSON-like documents. SQL databases, like MySQL, are relational databases with a fixed schema.

13. Varchar vs Varchar2 in MongoDB:

    • MongoDB doesn't have predefined data types like VARCHAR. It stores data as BSON, where the type is implicitly defined based on the data.

14. Triggers in MongoDB:

    • MongoDB does not have traditional triggers. You typically handle actions on data changes in the application layer.

15. ACID properties in MongoDB:

    • MongoDB is designed to provide high availability and scalability, and while it supports some ACID properties, it may relax consistency under certain scenarios to achieve performance.

It's important to note that MongoDB has its own set of concepts and best practices, and it may not align exactly with the relational database concepts mentioned in your original list. If you have more specific questions about MongoDB or any of the topics, feel free to ask!

Sure, let's use a simplified analogy to explain MySQL sharding and replication in layman's terms:

MySQL Replication (Replica Set):

Imagine you have a team of people working on a project, and there's one main person who keeps track of all the changes (let's call them the "Captain"). Whenever someone on the team makes a change or updates a document, the Captain takes note of it and informs everyone else. This way, everyone has the same information, and if one person is busy, others can step in and help.

• Example:
• Captain (Main Server): Keeps track of all changes.
• Team Members (Replica Servers): Receive updates from the Captain, ensuring they all have the same information.

MySQL Sharding:

Now, let's say your project becomes so massive that it's challenging for one person (or server) to handle everything. So, you decide to divide the project into sections, and each section is managed by a different person (or server). Each person (or server) only needs to focus on their specific section, making it more manageable.

• Example:
• Project (Database): Divided into sections (Shards).
• Each Section (Shard): Managed by a different person (Server).
• Queries (Requests): If someone needs information, they ask the person responsible for that specific section (Shard).

In summary:

• Replication (Replica Set): One main server keeps track of changes and informs other servers, ensuring everyone has the same information.
• Sharding: Breaking down a large project into smaller, manageable sections, with each section managed by a different server.

These are simplified explanations, and in real-world scenarios, implementing replication and sharding involves more technical details and considerations.

Certainly! Let's draw a comparison between Replica Set and Sharding concepts in MySQL and MongoDB in simple terms:

Replica Set: copy

MySQL:

• Analogy: Imagine you have a main copy of your project notes (master), and you make duplicate copies (replicas) for others in case the main copy goes missing.
• Explanation:
• In MySQL, a Replica Set is like having a master server and multiple replica servers.
• The master server handles changes to the data, and replicas copy those changes. This provides data redundancy and allows for better read scalability.

MongoDB:

• Analogy: Think of having a main notebook where you keep track of all the changes, and other team members have the same notebook to stay updated.
• Explanation:
• In MongoDB, a Replica Set consists of a primary server and secondary servers (replicas).
• The primary server manages changes, and the replicas replicate those changes. This ensures data availability and fault tolerance.

Sharding: divided

MySQL:

• Analogy: If your project grows too big for one person to handle, you divide it into sections, and each person is responsible for a specific section.
• Explanation:
• In MySQL, Sharding involves dividing a large database into smaller parts (shards).
• Each shard is managed by a separate MySQL server. This helps distribute the load and improves both read and write scalability.

MongoDB:

• Analogy: Imagine having a big library, and each section of books is placed on a different shelf, managed by a different librarian.
• Explanation:
• In MongoDB, Sharding means dividing the data into smaller chunks (shards).
• Each shard is a separate MongoDB server, handling its portion of the data. This enhances horizontal scaling and allows for efficient data distribution.

Key Differences:

• Replica Set: Focuses on redundancy, fault tolerance, and improved read scalability.
• Sharding: Focuses on distributing data for better write and read scalability.

In summary, both MySQL and MongoDB implement Replica Set and Sharding concepts, but the specifics of their implementation may differ. These concepts provide solutions for enhancing the performance, scalability, and reliability of database systems.

Certainly! Here are concise answers to your MySQL-related questions:

1. What is MySQL?

   • MySQL is an open-source relational database management system (RDBMS) that stores and manages data in tabular form.

2. Why is MySQL so popular?

   • MySQL is popular due to its open-source nature, ease of use, reliability, and strong community support.

3. What are the tables in MySQL? Explain the types.

   • Tables in MySQL organize data. Types include MyISAM (fast read, write), InnoDB (ACID compliance), MEMORY (temporary), and more.

4. Write a query for column addition in MySQL:

   • ALTER TABLE table_name ADD COLUMN column_name datatype;

5. What is a foreign key? Write a query to implement the same in MySQL.

   • A foreign key links tables based on a column. Query: ALTER TABLE table_name ADD FOREIGN KEY (column_name) REFERENCES another_table(another_column);

6. What is MySQL Workbench?

   • MySQL Workbench is a visual tool to design, develop, and manage MySQL databases.

7. How does database import/export work in MySQL?

   • Import: mysql -u username -p dbname < file.sql
   • Export: mysqldump -u username -p dbname > file.sql

8. How can we delete a column or a row in MySQL?

   • Delete column: ALTER TABLE table_name DROP COLUMN column_name;
   • Delete row: DELETE FROM table_name WHERE condition;

9. What are the different ways to join tables in MySQL?

   • Types include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.

10. Can a primary key be dropped in MySQL? If yes, how?

    • Yes, using: ALTER TABLE table_name DROP PRIMARY KEY;

11. What are Procedures in MySQL?

    • Procedures are sets of SQL statements stored in the database and executed as a single unit.

12. What is a trigger in MySQL?

    • A trigger is a set of instructions that automatically perform an action in response to certain events on a particular table.

13. How to add users in MySQL?

    • CREATE USER 'username'@'host' IDENTIFIED BY 'password';

14. What is the core difference between Oracle and MySQL?

    • Oracle is a commercial RDBMS, while MySQL is an open-source RDBMS.

15. What is CHAR and VARCHAR in MySQL?

    • CHAR is fixed-length, VARCHAR is variable-length. VARCHAR is often more efficient for storage.

16. Which drivers are necessary for MySQL?

    • JDBC (Java Database Connectivity) drivers are commonly used for MySQL.

17. What is a LIKE statement? Explain % and _ in LIKE.

    • LIKE is used for pattern matching. % matches any sequence of characters, _ matches any single character.

18. How to convert timestamps to date in MySQL?

    • Use FROM_UNIXTIME(timestamp).

19. Can a query be written in any case in MySQL?

    • Yes, MySQL is case-insensitive for queries.

20. How to save images in MySQL?

    • Store images in BLOB (Binary Large Object) columns.

21. How to get multiple condition results from data in MySQL?

    • Use the AND and OR operators in the WHERE clause.

22. What are the different file formats used by MyISAM?

    • MyISAM uses .MYD (data), .MYI (index), and .frm (table definition) files.

23. How does DISTINCT work in MySQL?

    • DISTINCT removes duplicate values from the result set.

24. Is there any upper limit for the number of columns in a table?

    • The theoretical limit is 4096 columns, but practical considerations may limit this.

25. What are Access Control Lists or ACLs, in accordance with MySQL?

    • ACLs in MySQL control user access to databases and tables.

26. How to make connections persistent in MySQL?

    • Use the mysql_pconnect function in PHP.

27. Explain the SAVEPOINT statement in MySQL.

    • SAVEPOINT sets a point within a transaction to which you can later roll back.

28. Can MySQL store images and videos?

    • Yes, using BLOB data types.

29. How does MySQL use indexes?

    • Indexes speed up query performance by enabling faster data retrieval.

30. Why is MySQL better than other databases?

    • MySQL is often preferred for its open-source nature, ease of use, and strong community support.

31. Where does MySQL store passwords?

    • In the user table of the mysql database.

32. Are MySQL and SQL servers the same?

    • No, MySQL is a specific type of SQL database management system.

33. Are MySQL databases encrypted?

    • MySQL supports encryption at different levels, including data at rest and data in transit.

34. Can MySQL store JSON?

    • Yes, MySQL has native support for JSON data types.

35. Why did MySQL shut down unexpectedly?

    • This could be due to various reasons, such as server issues, configuration problems, or resource constraints.

36. Are MySQL and MariaDB the same?

    • MariaDB is a fork of MySQL, aiming to maintain open-source freedom.

37. Why is my MySQL not working?

    • Check configuration files, server status, and logs for errors.

38. When does MySQL lock tables?

    • MySQL may lock tables during write operations to maintain data integrity.

39. What is the difference between MyISAM Static and MyISAM Dynamic?

    • MyISAM Static has a fixed-length data format, while MyISAM Dynamic allows variable-length rows.

40. What is an SQL Server?

    • SQL Server is a relational database management system developed by Microsoft.

These answers provide concise information on various aspects of MySQL. If you need further clarification on any topic, feel free to ask!

Certainly! Here are concise answers to your MongoDB-related questions:

1. What is MongoDB?

   • MongoDB is a NoSQL document-oriented database that stores data in flexible, JSON-like documents.

2. Why is MongoDB so popular?

   • MongoDB is popular for its flexibility, scalability, and ease of handling diverse data types. It's well-suited for applications with rapidly changing requirements.

3. What are the collections in MongoDB? Explain the types.

   • Collections in MongoDB are similar to tables in relational databases. There are no predefined types; collections can store documents with varying structures.

4. Write a query for a document addition in MongoDB:

   • db.collectionName.insert({ key: value });

5. What is a foreign key? Write a query to implement the same in MongoDB.

   • MongoDB uses embedding or referencing instead of traditional foreign keys. For referencing: { author: ObjectId("authorId") }.

6. What is MongoDB Compass?

   • MongoDB Compass is a graphical user interface (GUI) tool for MongoDB, providing a visual way to interact with databases.

7. How does database import/export work in MongoDB?

   • Import: mongoimport --db dbname --collection collectionName --file fileName.json
   • Export: mongoexport --db dbname --collection collectionName --out fileName.json

8. How can we delete a document in MongoDB?

   • db.collectionName.deleteOne({ key: value }); or db.collectionName.deleteMany({ key: value });

9. What are the different ways to join collections in MongoDB?

   • MongoDB uses $lookup for performing left outer joins between collections.

10. Can a primary key be dropped in MongoDB?

    • MongoDB's _id field acts as the primary key. While it can't be dropped, you can modify its value in some cases.

11. What are Procedures in MongoDB?

    • MongoDB does not have stored procedures like traditional relational databases.

12. What is a trigger in MongoDB?

    • MongoDB does not have triggers. Triggers are typically handled in the application layer.

13. How to add users in MongoDB?

    • use admin; db.createUser({ user: "username", pwd: "password", roles: ["readWrite"] });

14. What is the core difference between MongoDB and MySQL?

    • MongoDB is a NoSQL document-oriented database, while MySQL is a traditional relational database management system.

15. What is BSON?

    • BSON (Binary JSON) is a binary-encoded serialization format used by MongoDB to store and exchange data.

16. Which drivers are necessary for MongoDB?

    • MongoDB has official drivers for various programming languages like Node.js, Python, Java, etc.

17. What is a $regex operator in MongoDB? Explain % and _ in $regex.

    • $regex is used for pattern matching. % and _ are not used in MongoDB regex; instead, you use .* for any character and . for a single character.

18. How to convert timestamps to date in MongoDB?

    • Use $toDate aggregation operator: { $toDate: "$timestampField" }.

19. Can a query be written in any case in MongoDB?

    • Yes, MongoDB is case-insensitive for queries.

20. How to save images in MongoDB?

    • Store images as binary data (BSON binary type) or use GridFS for large files.

21. How to get multiple condition results from data in MongoDB?

    • Use $and or $or operators in the find query.

22. How does DISTINCT work in MongoDB?

    • Use distinct on a specific field: db.collectionName.distinct("fieldName");

23. Is there any upper limit for the number of fields in a document?

    • MongoDB has a BSON document size limit (16 MB), so extremely large documents may encounter issues.

24. How does MongoDB use indexes?

    • MongoDB uses indexes to improve query performance. You can create indexes on specific fields.

25. Can MongoDB store images and videos?

    • Yes, MongoDB can store images and videos as binary data or using GridFS for larger files.

26. Why is MongoDB better than other databases?

    • MongoDB's flexibility, scalability, and ease of use make it suitable for various types of applications.

27. How does MongoDB handle ACID properties?

    • MongoDB sacrifices strict ACID properties for performance and scalability, offering eventual consistency instead.

28. Where does MongoDB store passwords?

    • Passwords are stored securely in the MongoDB user collection.

29. Are MongoDB and SQL servers the same?

    • No, MongoDB is a NoSQL document-oriented database, while SQL Server is a relational database management system.

30. Are MongoDB databases encrypted?

    • MongoDB supports encryption at rest and in transit to secure data.

31. Can MongoDB store JSON?

    • Yes, MongoDB stores data in BSON format, which is similar to JSON.

32. Why did MongoDB shut down unexpectedly?

    • Various reasons, such as system issues, configuration problems, or resource constraints.

33. Are MongoDB and WiredTiger the same?

    • WiredTiger is the default storage engine for MongoDB, providing features like compression and concurrency control.

34. Can MongoDB store geospatial data?

    • Yes, MongoDB has built-in support for geospatial indexing and queries.

35. What is the Mongoose library in MongoDB?

    • Mongoose is an ODM (Object Data Modeling) library for MongoDB and Node.js.