Entity Framework (EF) is an open-source Object-Relational Mapper (ORM) for .NET applications. It allows developers to interact with databases using .NET objects, eliminating much of the need for direct SQL queries.

2. Explain advantage of Entity Framework over ADO.NET

Here are the key advantages of using Entity Framework (EF) over ADO.NET:

1. Simplified Data Access

Entity Framework: EF abstracts the database interactions into high-level, object-oriented operations. Developers work with .NET objects and collections, making data access more intuitive and reducing the amount of boilerplate code.
ADO.NET: Requires explicit management of database connections, commands, readers, and manually mapping data from the database to application objects, which can be cumbersome and error-prone.

2. Productivity and Development Speed

Entity Framework: Code-First and Database-First approaches allow for automatic generation of data models and database schema. The use of LINQ (Language Integrated Query) for querying the database is more readable and concise compared to writing raw SQL.
ADO.NET: Developers must write and manage SQL queries and data mapping code, which can be time-consuming and lead to more bugs.

3. Maintainability

Entity Framework: Changes in the data model are easily managed through EF migrations, which automatically update the database schema. This ensures consistency between the codebase and the database.
ADO.NET: Any changes in the database schema require manual updates to the SQL queries and data mapping code, increasing the maintenance burden.

4. Automatic Change Tracking

Entity Framework: EF automatically tracks changes made to objects retrieved from the database. When SaveChanges is called, EF determines the changes and generates the appropriate SQL commands to update the database.
ADO.NET: Developers must manually track changes and construct the necessary SQL commands to reflect these changes in the database.

5. Built-in Relationship Management

Entity Framework: EF inherently understands and manages relationships between entities, such as one-to-one, one-to-many, and many-to-many relationships, making it easier to work with related data.
ADO.NET: Relationships must be managed manually through joins in SQL queries and additional code to handle related data.

6. Query Composability

Entity Framework: Queries can be composed using LINQ, which allows combining and reusing query logic in a readable and maintainable way. The generated SQL is optimized for the database provider in use.
ADO.NET: Queries are often written as strings in SQL, which can be less readable and harder to compose or reuse.

7. Database Provider Independence

Entity Framework: EF supports multiple database providers (e.g., SQL Server, MySQL, PostgreSQL), allowing for easier switching between different databases without changing the data access code.
ADO.NET: Typically, code is more tightly coupled to a specific database provider, and switching databases requires significant changes to the data access code.

8. Enhanced Security

Entity Framework: EF's use of parameterized queries by default helps prevent SQL injection attacks. It also abstracts the SQL details away from the developer, reducing the risk of introducing security vulnerabilities.
ADO.NET: While parameterized queries can be used, developers must remember to implement them correctly to avoid security issues like SQL injection.

9. Rich Data Validation and Business Logic Integration

Entity Framework: EF supports data annotations and the Fluent API to enforce validation rules and business logic directly within the model classes, ensuring consistency and reducing the need for additional validation code.
ADO.NET: Validation and business logic must be implemented manually, often leading to duplicated effort and potential inconsistencies.

In summary, Entity Framework provides a higher level of abstraction and automation for database operations, which can significantly improve productivity, maintainability, and security. While ADO.NET offers fine-grained control and might be preferred in certain high-performance scenarios, EF is generally the better choice for most application development due to its ease of use and comprehensive features.

3. Explain Code First, Model First and Database First approaches in Entity Framework

Code-First: You define the data models using C# or VB.NET classes, and EF generates the database schema based on these models.

Model-First: You design the model using a visual designer, which then generates the database schema and the corresponding classes.

Database-First: You start with an existing database, and EF generates the classes that map to the database tables.

4. What is the DBContext class in Entity Framework ?

The DbContext class is a central part of Entity Framework (EF) and serves as the primary class for interacting with the database. It provides a range of functionalities to manage database connections, perform CRUD (Create, Read, Update, Delete) operations, track changes, and handle transactions. Here’s a detailed look at the DbContext class:

1. Database Connection Management

DbContext manages the connection to the database. It can be configured to use a specific database connection string, which can be set in the application’s configuration file or directly in the code.

2. Querying and CRUD Operations

It allows you to query the database using LINQ (Language Integrated Query) and perform CRUD operations on the data. The DbSet properties represent collections of entities, which can be queried and updated.

3. Change Tracking

DbContext automatically tracks changes made to the entities that are retrieved from the database. When SaveChanges is called, it determines what changes have been made (insertions, deletions, updates) and generates the appropriate SQL commands to persist these changes to the database.

4. Relationship Management

It manages the relationships between entities, ensuring that related data is loaded and managed correctly. This includes handling one-to-one, one-to-many, and many-to-many relationships.

5. Data Validation

DbContext supports validation of data through data annotations and the Fluent API. This ensures that data integrity rules are enforced before the data is persisted to the database.

6. Migrations and Schema Management

DbContext works with EF Migrations to handle database schema changes over time. It allows you to apply, rollback, and script migrations to update the database schema as your model evolves.

7. Configurations

Configurations for the DbContext can be defined in the OnModelCreating method using the Fluent API. This method is overridden to configure the model and relationships using code.

public class ApplicationDbContext : DbContext
{
    public DbSet<Customer> Customers { get; set; }
    public DbSet<Order> Orders { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("DatabaseConnectionString");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Customer>()
            .Property(c => c.Name)
            .IsRequired()
            .HasMaxLength(50);

        modelBuilder.Entity<Order>()
            .HasOne(o => o.Customer)
            .WithMany(c => c.Orders)
            .HasForeignKey(o => o.CustomerId);
    }
}

public class Customer
{
    public int CustomerId { get; set; }
    public string Name { get; set; }
    public ICollection<Order> Orders { get; set; }
}

public class Order
{
    public int OrderId { get; set; }
    public DateTime OrderDate { get; set; }
    public int CustomerId { get; set; }
    public Customer Customer { get; set; }
}

5. What is DBSet ?

A DbSet represents the collection of all entities in the context, or that can be queried from the database, of a given type. For example, if you have a DbSet<Customer>, it represents all the customers that you can work with in your application.

DbSet provides methods to query the database. You can use LINQ to Entities to build queries against the data set. This allows you to perform filtering, sorting, and projection operations.

using (var context = new ApplicationDbContext())
{
    // Adding a new customer
    var customer = new Customer { Name = "John Doe" };
    context.Customers.Add(customer);
    context.SaveChanges();

    // Querying customers
    var customers = context.Customers.ToList();

    // Updating a customer
    var existingCustomer = context.Customers.Find(1);
    existingCustomer.Name = "John Smith";
    context.SaveChanges();

    // Deleting a customer
    context.Customers.Remove(existingCustomer);
    context.SaveChanges();
}

6. What is the advantage of using LINQ ?

LINQ queries are written in a syntax that is integrated with the programming language (C# or VB.NET), making the code more readable and easier to maintain.

LINQ queries are checked at compile-time for syntax errors and type safety, reducing the risk of runtime errors.

LINQ queries support deferred execution, meaning that the query is not executed until the data is actually accessed. This can improve performance by avoiding unnecessary data retrieval.

7. What is the difference between SQL and NoSQL ?

SQL databases, like MySQL and PostgreSQL, use structured tables with fixed schemas and support complex queries using SQL.

In contrast, NoSQL databases, like MongoDB and Cassandra, offer flexible, schema-less data models, making them suitable for unstructured data.

8. What is the difference between Views, Stored Procedure and Function ?

Stored Procedure

A stored procedure is a precompiled collection of SQL statements and procedural logic stored in the database.It allows you to group and organize SQL statements into a single unit of work that can be executed repeatedly. Stored procedures are commonly used for complex data manipulation, business logic implementation, and to encapsulate frequently performed tasks.

View

A view is a virtual table generated as the result of a SELECT query stored in the database. It provides a way to simplify complex queries and encapsulate them into reusable objects. Views are commonly used to abstract complex joins, apply security filters, and present data in a structured format. It can be queried like a table, but the underlying data is not physically stored; instead, it's generated dynamically when the view is queried.

Function

A function is a reusable set of SQL statements that can accept parameters, perform computations, and return a single value or a table variable. It provides a way to encapsulate common computations or transformations into reusable modules. Functions are commonly used in calculations, data transformations, and as part of queries to simplify complex expressions.It can accept input parameters, perform calculations or data manipulations, and return a single value or a result set.

9. What is the difference between varchar and nvarchar ?

Nvarchar stores UNICODE data. If you have requirements to store UNICODE or multilingual data, nvarchar is the choice. Varchar stores ASCII data and should be your data type of choice for normal use.

For nvarchar, when using characters defined in the Unicode range 0-65,535, one character can be stored per each byte-pair, however, in higher Unicode ranges (65,536-1,114,111) one character may use two byte-pairs. Whereas varchar only uses 1 byte.

10. What is the difference between primary key and unique key ?

Null Values: Primary Key columns cannot contain NULL values, while Unique Key columns can allow NULL values (with certain restrictions).

Number of Keys: Each table can have only one Primary Key, but multiple Unique Key constraints can be defined in a table.

Indexing: Primary Key constraints typically create a clustered index, while Unique Key constraints do not automatically create any index (though you can manually create indexes).

Usage: Primary Keys are used to uniquely identify each row and establish relationships between tables, while Unique Keys are used to enforce uniqueness but are not necessarily used for identification.

11. What is the difference between DELETE, DROP and TRUNCATE ?

DELETE is used to remove specific rows from a table based on conditions, while TRUNCATE removes all rows from a table.

DROP is used to remove entire database objects, including tables, views, and stored procedures, from the database.

DELETE and TRUNCATE can be rolled back (if used within a transaction), while DROP is irreversible.

TRUNCATE is generally faster than DELETE, especially for large tables, as it doesn't log individual row deletions.

12. What is the difference between UNION and UNION ALL in SQL ?

UNION eliminates duplicate rows from the combined result set, presenting unique records, while UNION ALL remains all rows, including duplicates.

13. What is the use of indexing in SQL ?

Indexes are special data structures that improve the speed of data retrieval operations on a table at the cost of additional writes and storage space. They are used to quickly locate data without having to search every row in a database table.

14. What is a deadlock and how to avoid it ?

A deadlock in SQL occurs when two or more transactions are waiting for each other to release locks on resources, resulting in a cycle of dependencies that prevents any of the transactions from proceeding. Deadlocks can significantly impact database performance and transaction throughput.

Example of a Deadlock

Transaction A locks Resource 1 and needs Resource 2 to proceed.

Transaction B locks Resource 2 and needs Resource 1 to proceed.

Both transactions wait indefinitely for the other to release its lock, causing a deadlock.

Avoiding deadlocks involves strategies like accessing resources in a consistent order, keeping transactions short, using appropriate isolation levels, and implementing retry logic.

15. Explain ACID properties

ACID properties are a set of principles that ensure reliable processing of database transactions. They stand for Atomicity, Consistency, Isolation, and Durability. Here's a brief explanation of each property:

1. Atomicity

Definition: Ensures that a transaction is treated as a single unit, which either completely succeeds or completely fails.
Example: If a bank transfer involves debiting one account and crediting another, atomicity ensures that either both actions are completed or neither is, preventing partial updates.

2. Consistency

Definition: Ensures that a transaction takes the database from one valid state to another, maintaining all predefined rules and constraints.
Example: After a transaction, all constraints (like unique keys, foreign keys) must still hold true. For instance, transferring money should not create money from nowhere or lose money.

3. Isolation

Definition: Ensures that concurrent transactions do not interfere with each other, maintaining data consistency. Transactions appear to be executed in isolation.
Example: If two transactions are updating the same set of rows, isolation ensures that each transaction sees a consistent view of the data and the results of the transactions do not conflict.

4. Durability

Definition: Ensures that once a transaction has been committed, it will remain so, even in the case of a system failure.
Example: Once a bank transfer is completed and the transaction is committed, the changes should persist even if there is a system crash immediately after the transaction.

Detailed Example

Consider a bank transfer scenario where $100 is transferred from Michael’s account to Jim’s account.

Atomicity: The transaction will ensure that the $100 is either fully debited from Michael’s account and credited to Jim’s account or neither action will occur. There’s no partial state where Michael’s account is debited, but Jim’s account is not credited.
Consistency: The transaction will ensure that the total amount of money in both accounts before and after the transfer remains consistent with the bank’s rules. If Michael’s account had $500 and Jim’s account had $300, after the transfer, Michael should have $400 and Jim should have $400, preserving the total balance.
Isolation: If another transaction is occurring simultaneously, such as another transfer involving Michael or Jim, each transaction will operate as if it is the only transaction in the system, ensuring that the final balances are correct and transactions do not interfere with each other.
Durability: Once the transaction is completed and committed, the changes will persist in the database. If there’s a power failure right after the commit, the database will reflect that $100 was successfully transferred from Michael to Jim once the system is back online.

Summary

The ACID properties are fundamental to ensure reliable, consistent, and robust database transactions. By adhering to these properties, databases maintain data integrity and reliability even in the face of errors, concurrent operations, and system failures.

16. What are triggers in SQL ?

In SQL, triggers are special types of stored procedures that automatically execute in response to specific events occurring in the database. These events include insertions, updates, and deletions of data in tables. Triggers can be used to enforce business rules, maintain data integrity, or automate tasks.

Conclusion

In conclusion, understanding Entity Framework and SQL is crucial for building robust and efficient database-driven applications.

Entity Framework provides a higher level of abstraction and productivity compared to ADO.NET, simplifying data access and management.

SQL databases offer a structured and reliable way to store and retrieve data, while NoSQL databases provide flexibility and scalability for diverse data needs.

Knowing the differences between SQL and NoSQL, as well as the ACID properties, triggers, and optimization techniques, empowers developers to design and implement effective database solutions.

By mastering these concepts and best practices, developers can ensure data integrity, performance, and scalability in their applications, making them well-equipped for success in any SQL and Entity Framework interviews and also real-world scenarios.

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