using System; using System.Data; using System.Collections.Generic; using System.Threading.Tasks; // Requires NuGet packages: // Microsoft.Data.SqlClient // Microsoft.Data.Sqlite // MySql.Data // Npgsql // Dapper // Microsoft.Extensions.Configuration.EnvironmentVariables (for .NET environment variables) namespace RelationalDatabaseExamples { ///

/// Examples of working with relational databases in C# ///

class Program { // Configuration for environment variables private static readonly EnvironmentVariableManager _env = new EnvironmentVariableManager(); static async Task Main(string[] args) { Console.WriteLine("C# Relational Database Examples"); Console.WriteLine("==============================="); try { // SQLite example (works without additional setup) await SQLiteExample(); // The following examples require database servers to be running // Uncomment them if you have the necessary databases set up // await SqlServerExample(); // await MySqlExample(); // await PostgreSqlExample(); // await DapperExample(); Console.WriteLine("\nAll examples completed successfully."); } catch (Exception ex) { Console.WriteLine($"Error: {ex.Message}"); } } ///

/// Example using SQLite ///

static async Task SQLiteExample() { Console.WriteLine("\n=== SQLite Example ==="); // Make sure you have the Microsoft.Data.Sqlite NuGet package installed var connectionString = _env.GetValue("SQLITE_CONNECTION_STRING", "Data Source=:memory:"); using (var connection = new Microsoft.Data.Sqlite.SqliteConnection(connectionString)) { await connection.OpenAsync(); Console.WriteLine("Connected to SQLite database"); // Create a table using (var command = connection.CreateCommand()) { command.CommandText = @" CREATE TABLE Employees ( Id INTEGER PRIMARY KEY, Name TEXT NOT NULL, Department TEXT NOT NULL, Salary REAL, HireDate TEXT )"; await command.ExecuteNonQueryAsync(); Console.WriteLine("Created Employees table"); } // Insert data using parameters using (var command = connection.CreateCommand()) { command.CommandText = @" INSERT INTO Employees (Name, Department, Salary, HireDate) VALUES (@name, @department, @salary, @hireDate)"; command.Parameters.AddWithValue("@name", "John Smith"); command.Parameters.AddWithValue("@department", "Engineering"); command.Parameters.AddWithValue("@salary", 85000.00); command.Parameters.AddWithValue("@hireDate", DateTime.Now.ToString("yyyy-MM-dd")); await command.ExecuteNonQueryAsync(); Console.WriteLine("Inserted employee: John Smith"); // Insert another employee command.Parameters.Clear(); command.Parameters.AddWithValue("@name", "Jane Doe"); command.Parameters.AddWithValue("@department", "Marketing"); command.Parameters.AddWithValue("@salary", 75000.00); command.Parameters.AddWithValue("@hireDate", DateTime.Now.AddDays(-90).ToString("yyyy-MM-dd")); await command.ExecuteNonQueryAsync(); Console.WriteLine("Inserted employee: Jane Doe"); // One more employee command.Parameters.Clear(); command.Parameters.AddWithValue("@name", "Bob Johnson"); command.Parameters.AddWithValue("@department", "Engineering"); command.Parameters.AddWithValue("@salary", 82000.00); command.Parameters.AddWithValue("@hireDate", DateTime.Now.AddDays(-180).ToString("yyyy-MM-dd")); await command.ExecuteNonQueryAsync(); Console.WriteLine("Inserted employee: Bob Johnson"); } // Query all employees Console.WriteLine("\nAll employees:"); using (var command = connection.CreateCommand()) { command.CommandText = "SELECT Id, Name, Department, Salary FROM Employees"; using (var reader = await command.ExecuteReaderAsync()) { while (await reader.ReadAsync()) { Console.WriteLine($"ID: {reader.GetInt32(0)}, Name: {reader.GetString(1)}, " + $"Department: {reader.GetString(2)}, Salary: ${reader.GetDouble(3):N2}"); } } } // Filtered query with parameters Console.WriteLine("\nEngineering department employees:"); using (var command = connection.CreateCommand()) { command.CommandText = "SELECT Name, Salary FROM Employees WHERE Department = @department"; command.Parameters.AddWithValue("@department", "Engineering"); using (var reader = await command.ExecuteReaderAsync()) { while (await reader.ReadAsync()) { Console.WriteLine($"{reader.GetString(0)}: ${reader.GetDouble(1):N2}"); } } } // Aggregate query Console.WriteLine("\nDepartment statistics:"); using (var command = connection.CreateCommand()) { command.CommandText = @" SELECT Department, COUNT(*) as EmployeeCount, AVG(Salary) as AvgSalary, SUM(Salary) as TotalSalary FROM Employees GROUP BY Department"; using (var reader = await command.ExecuteReaderAsync()) { while (await reader.ReadAsync()) { Console.WriteLine($"{reader.GetString(0)}: " + $"{reader.GetInt32(1)} employees, " + $"Avg: ${reader.GetDouble(2):N2}, " + $"Total: ${reader.GetDouble(3):N2}"); } } } // Transaction example await using (var transaction = connection.BeginTransaction()) { try { // Update salary in a transaction using (var command = connection.CreateCommand()) { command.Transaction = transaction; command.CommandText = "UPDATE Employees SET Salary = Salary * 1.1 WHERE Department = @department"; command.Parameters.AddWithValue("@department", "Engineering"); var rowsAffected = await command.ExecuteNonQueryAsync(); Console.WriteLine($"\nGave 10% raise to {rowsAffected} engineering employees"); } // Verify the changes using (var command = connection.CreateCommand()) { command.Transaction = transaction; command.CommandText = "SELECT Name, Salary FROM Employees WHERE Department = @department"; command.Parameters.AddWithValue("@department", "Engineering"); Console.WriteLine("Updated engineering salaries:"); using (var reader = await command.ExecuteReaderAsync()) { while (await reader.ReadAsync()) { Console.WriteLine($"{reader.GetString(0)}: ${reader.GetDouble(1):N2}"); } } } // Commit the transaction await transaction.CommitAsync(); Console.WriteLine("Transaction committed"); } catch (Exception ex) { await transaction.RollbackAsync(); Console.WriteLine($"Transaction rolled back: {ex.Message}"); } } } } ///

/// Example using SQL Server ///

static async Task SqlServerExample() { Console.WriteLine("\n=== SQL Server Example ==="); Console.WriteLine("To run this example, you need SQL Server and the Microsoft.Data.SqlClient NuGet package."); // Connection string for SQL Server from environment variables var connectionString = _env.GetValue("SQLSERVER_CONNECTION_STRING", "Server=localhost;Database=TestDB;Trusted_Connection=True;"); Console.WriteLine($"Using connection info from environment: {GetSanitizedConnectionString(connectionString)}"); using (var connection = new Microsoft.Data.SqlClient.SqlConnection(connectionString)) { await connection.OpenAsync(); Console.WriteLine("Connected to SQL Server"); // Create a table with SQL Server specific features using (var command = connection.CreateCommand()) { command.CommandText = @" IF NOT EXISTS (SELECT * FROM sys.tables WHERE name = 'Products') BEGIN CREATE TABLE Products ( Id INT IDENTITY(1,1) PRIMARY KEY, Name NVARCHAR(100) NOT NULL, Description NVARCHAR(MAX), Price DECIMAL(10,2) NOT NULL, Category NVARCHAR(50), CreatedAt DATETIME2 DEFAULT GETDATE(), UpdatedAt DATETIME2, IsActive BIT DEFAULT 1 ); CREATE INDEX IX_Products_Category ON Products(Category); END"; await command.ExecuteNonQueryAsync(); Console.WriteLine("Created Products table"); } // Insert with output parameter to get the identity value using (var command = connection.CreateCommand()) { command.CommandText = @" INSERT INTO Products (Name, Description, Price, Category) OUTPUT INSERTED.Id VALUES (@name, @description, @price, @category)"; command.Parameters.AddWithValue("@name", "Laptop"); command.Parameters.AddWithValue("@description", "High-performance laptop"); command.Parameters.AddWithValue("@price", 1299.99); command.Parameters.AddWithValue("@category", "Electronics"); var productId = (int)await command.ExecuteScalarAsync(); Console.WriteLine($"Inserted product with ID: {productId}"); } // SQL Server specific features: CTE, ROW_NUMBER using (var command = connection.CreateCommand()) { command.CommandText = @" WITH RankedProducts AS ( SELECT Name, Price, Category, ROW_NUMBER() OVER (PARTITION BY Category ORDER BY Price DESC) AS PriceRank FROM Products ) SELECT Name, Price, Category, PriceRank FROM RankedProducts WHERE PriceRank <= 3"; using (var reader = await command.ExecuteReaderAsync()) { Console.WriteLine("\nTop 3 most expensive products per category:"); while (await reader.ReadAsync()) { Console.WriteLine($"{reader.GetString(0)} - ${reader.GetDecimal(1):N2} - " + $"{reader.GetString(2)} (Rank: {reader.GetInt64(3)})"); } } } // Using stored procedure using (var command = connection.CreateCommand()) { // First create the stored procedure command.CommandText = @" IF NOT EXISTS (SELECT * FROM sys.procedures WHERE name = 'GetProductsByCategory') BEGIN EXEC(' CREATE PROCEDURE GetProductsByCategory @CategoryName NVARCHAR(50), @MinPrice DECIMAL(10,2) = 0 AS BEGIN SELECT Id, Name, Price FROM Products WHERE Category = @CategoryName AND Price >= @MinPrice ORDER BY Price DESC; END ') END"; await command.ExecuteNonQueryAsync(); Console.WriteLine("Created stored procedure"); // Use the stored procedure command.CommandText = "GetProductsByCategory"; command.CommandType = CommandType.StoredProcedure; command.Parameters.Clear(); command.Parameters.AddWithValue("@CategoryName", "Electronics"); command.Parameters.AddWithValue("@MinPrice", 500); using (var reader = await command.ExecuteReaderAsync()) { Console.WriteLine("\nElectronics products over $500:"); while (await reader.ReadAsync()) { Console.WriteLine($"ID: {reader.GetInt32(0)}, Name: {reader.GetString(1)}, " + $"Price: ${reader.GetDecimal(2):N2}"); } } } } } ///

/// Example using MySQL ///

static async Task MySqlExample() { Console.WriteLine("\n=== MySQL Example ==="); Console.WriteLine("To run this example, you need MySQL and the MySql.Data NuGet package."); // Connection string for MySQL from environment variables var connectionString = _env.GetValue("MYSQL_CONNECTION_STRING", "Server=localhost;Database=testdb;Uid=root;Pwd=password;"); Console.WriteLine($"Using connection info from environment: {GetSanitizedConnectionString(connectionString)}"); using (var connection = new MySql.Data.MySqlClient.MySqlConnection(connectionString)) { await connection.OpenAsync(); Console.WriteLine("Connected to MySQL"); // Create a table with MySQL specific features using (var command = connection.CreateCommand()) { command.CommandText = @" CREATE TABLE IF NOT EXISTS Orders ( id INT AUTO_INCREMENT PRIMARY KEY, customer_name VARCHAR(100) NOT NULL, total DECIMAL(10,2) NOT NULL, status ENUM('pending', 'shipped', 'delivered') NOT NULL, created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP, updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, INDEX idx_status (status), INDEX idx_created_at (created_at) ) ENGINE=InnoDB"; await command.ExecuteNonQueryAsync(); Console.WriteLine("Created Orders table"); } // Insert with MySQL-specific AUTO_INCREMENT handling using (var command = connection.CreateCommand()) { command.CommandText = @" INSERT INTO Orders (customer_name, total, status) VALUES (@customerName, @total, @status)"; command.Parameters.AddWithValue("@customerName", "John Doe"); command.Parameters.AddWithValue("@total", 123.45); command.Parameters.AddWithValue("@status", "pending"); await command.ExecuteNonQueryAsync(); Console.WriteLine($"Inserted order, ID: {command.LastInsertedId}"); } // MySQL specific functions using (var command = connection.CreateCommand()) { command.CommandText = @" SELECT id, customer_name, total, status, DATE_FORMAT(created_at, '%Y-%m-%d %H:%i:%s') AS formatted_date, DATEDIFF(NOW(), created_at) AS days_since_creation FROM Orders WHERE created_at > DATE_SUB(NOW(), INTERVAL 7 DAY)"; using (var reader = await command.ExecuteReaderAsync()) { Console.WriteLine("\nRecent orders:"); while (await reader.ReadAsync()) { Console.WriteLine($"ID: {reader.GetInt32(0)}, " + $"Customer: {reader.GetString(1)}, " + $"Total: ${reader.GetDecimal(2):N2}, " + $"Status: {reader.GetString(3)}, " + $"Created: {reader.GetString(4)} " + $"({reader.GetInt32(5)} days ago)"); } } } } } ///

/// Example using PostgreSQL ///

static async Task PostgreSqlExample() { Console.WriteLine("\n=== PostgreSQL Example ==="); Console.WriteLine("To run this example, you need PostgreSQL and the Npgsql NuGet package."); // Connection string for PostgreSQL from environment variables var connectionString = _env.GetValue("POSTGRES_CONNECTION_STRING", "Host=localhost;Database=testdb;Username=postgres;Password=password"); Console.WriteLine($"Using connection info from environment: {GetSanitizedConnectionString(connectionString)}"); using (var connection = new Npgsql.NpgsqlConnection(connectionString)) { await connection.OpenAsync(); Console.WriteLine("Connected to PostgreSQL"); // Create a table with PostgreSQL-specific features using (var command = connection.CreateCommand()) { command.CommandText = @" CREATE TABLE IF NOT EXISTS products ( id SERIAL PRIMARY KEY, name VARCHAR(100) NOT NULL, description TEXT, price DECIMAL(10,2) NOT NULL, created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP, tags TEXT[], metadata JSONB )"; await command.ExecuteNonQueryAsync(); Console.WriteLine("Created products table"); } // Insert with PostgreSQL-specific data types using (var command = connection.CreateCommand()) { command.CommandText = @" INSERT INTO products (name, description, price, tags, metadata) VALUES (@name, @description, @price, @tags, @metadata) RETURNING id"; command.Parameters.AddWithValue("@name", "Laptop"); command.Parameters.AddWithValue("@description", "High-performance laptop"); command.Parameters.AddWithValue("@price", 999.99); command.Parameters.AddWithValue("@tags", new string[] { "electronics", "computers" }); command.Parameters.AddWithValue("@metadata", "{""brand"": ""TechBrand"", ""warranty"": ""2 years"", ""specs"": {""cpu"": ""i7"", ""ram"": ""16GB""}}"); var productId = (int)await command.ExecuteScalarAsync(); Console.WriteLine($"Inserted product with ID: {productId}"); } // PostgreSQL-specific features: JSONB, arrays using (var command = connection.CreateCommand()) { command.CommandText = @" SELECT name, price, metadata->>'brand' AS brand, metadata->'specs'->>'cpu' AS cpu, array_length(tags, 1) AS tag_count, extract(year from created_at) AS year FROM products WHERE 'electronics' = ANY(tags)"; using (var reader = await command.ExecuteReaderAsync()) { Console.WriteLine("\nElectronics products:"); while (await reader.ReadAsync()) { Console.WriteLine($"{reader.GetString(0)} - ${reader.GetDecimal(1):N2} - " + $"Brand: {reader.GetString(2)}, CPU: {reader.GetString(3)}, " + $"Tag count: {reader.GetInt32(4)}, Year: {reader.GetDouble(5)}"); } } } } } ///

/// Example using Dapper micro-ORM ///

static async Task DapperExample() { Console.WriteLine("\n=== Dapper Example ==="); Console.WriteLine("To run this example, you need the Dapper NuGet package and a database connection."); // Connection string from environment variables var connectionString = _env.GetValue("DAPPER_CONNECTION_STRING", "Data Source=:memory:"); using (var connection = new Microsoft.Data.Sqlite.SqliteConnection(connectionString)) { await connection.OpenAsync(); Console.WriteLine("Connected to SQLite for Dapper example"); // Set up database await connection.ExecuteAsync(@" CREATE TABLE Customers ( Id INTEGER PRIMARY KEY, Name TEXT NOT NULL, Email TEXT NOT NULL ); CREATE TABLE Orders ( Id INTEGER PRIMARY KEY, CustomerId INTEGER NOT NULL, Amount REAL NOT NULL, OrderDate TEXT NOT NULL, FOREIGN KEY (CustomerId) REFERENCES Customers (Id) ); CREATE TABLE OrderItems ( Id INTEGER PRIMARY KEY, OrderId INTEGER NOT NULL, ProductName TEXT NOT NULL, Quantity INTEGER NOT NULL, UnitPrice REAL NOT NULL, FOREIGN KEY (OrderId) REFERENCES Orders (Id) ); "); // Insert customers var customerIds = await connection.ExecuteAsync(@" INSERT INTO Customers (Name, Email) VALUES (@Name, @Email)", new[] { new { Name = "Alice Smith", Email = "alice@example.com" }, new { Name = "Bob Jones", Email = "bob@example.com" } }); Console.WriteLine($"Inserted {customerIds} customers"); // Insert orders var orderDate = DateTime.Now; var order1Id = await connection.QuerySingleAsync(@" INSERT INTO Orders (CustomerId, Amount, OrderDate) VALUES (@CustomerId, @Amount, @OrderDate) RETURNING Id", new { CustomerId = 1, Amount = 125.50, OrderDate = orderDate.ToString("yyyy-MM-dd") }); // Insert order items await connection.ExecuteAsync(@" INSERT INTO OrderItems (OrderId, ProductName, Quantity, UnitPrice) VALUES (@OrderId, @ProductName, @Quantity, @UnitPrice)", new[] { new { OrderId = order1Id, ProductName = "Keyboard", Quantity = 1, UnitPrice = 75.50 }, new { OrderId = order1Id, ProductName = "Mouse", Quantity = 1, UnitPrice = 50.00 } }); // Query with Dapper var customers = await connection.QueryAsync("SELECT * FROM Customers"); Console.WriteLine("\nAll customers:"); foreach (var customer in customers) { Console.WriteLine($"ID: {customer.Id}, Name: {customer.Name}, Email: {customer.Email}"); } // Query with join and multi-mapping Console.WriteLine("\nOrders with customer info:"); var orders = await connection.QueryAsync( @"SELECT o.Id, o.Amount, o.OrderDate, c.Id, c.Name, c.Email FROM Orders o JOIN Customers c ON o.CustomerId = c.Id", (order, customer) => { order.Customer = customer; return order; }, splitOn: "Id" ); foreach (var order in orders) { Console.WriteLine($"Order #{order.Id} - ${order.Amount:N2} on {order.OrderDate} " + $"by {order.Customer.Name} ({order.Customer.Email})"); } // Query order details with complex mapping Console.WriteLine("\nOrder details:"); var orderWithItems = await connection.QueryAsync( @"SELECT o.Id, o.Amount, o.OrderDate, i.Id, i.ProductName, i.Quantity, i.UnitPrice FROM Orders o JOIN OrderItems i ON o.Id = i.OrderId WHERE o.Id = @OrderId", (order, item) => { if (order.Items == null) order.Items = new List(); order.Items.Add(item); return order; }, new { OrderId = order1Id }, splitOn: "Id" ); var orderDetail = orderWithItems.GroupBy(o => o.Id).Select(g => { var order = g.First(); order.Items = g.Select(o => o.Items.Single()).ToList(); return order; }).First(); Console.WriteLine($"Order #{orderDetail.Id} - ${orderDetail.Amount:N2} on {orderDetail.OrderDate}"); foreach (var item in orderDetail.Items) { Console.WriteLine($" {item.ProductName}: {item.Quantity} x ${item.UnitPrice:N2} = ${item.Quantity * item.UnitPrice:N2}"); } // Execute scalar var totalSales = await connection.ExecuteScalarAsync( "SELECT SUM(Amount) FROM Orders"); Console.WriteLine($"\nTotal sales: ${totalSales:N2}"); } } ///

/// Sanitizes a connection string by hiding the password ///

private static string GetSanitizedConnectionString(string connectionString) { // Simple sanitization to hide password/credentials if (string.IsNullOrEmpty(connectionString)) return connectionString; // Handle different formats for different database providers return connectionString .Replace(GetPasswordPart(connectionString, "Password="), "Password=*****") .Replace(GetPasswordPart(connectionString, "Pwd="), "Pwd=*****") .Replace(GetPasswordPart(connectionString, "password="), "password=*****"); } private static string GetPasswordPart(string connectionString, string passwordPrefix) { int pwdIndex = connectionString.IndexOf(passwordPrefix, StringComparison.OrdinalIgnoreCase); if (pwdIndex < 0) return string.Empty; int startIndex = pwdIndex + passwordPrefix.Length; int endIndex = connectionString.IndexOf(';', startIndex); if (endIndex < 0) endIndex = connectionString.Length; return connectionString.Substring(pwdIndex, endIndex - pwdIndex); } // Model classes for Dapper example public class Customer { public int Id { get; set; } public string Name { get; set; } public string Email { get; set; } public List Orders { get; set; } } public class Order { public int Id { get; set; } public int CustomerId { get; set; } public double Amount { get; set; } public string OrderDate { get; set; } public Customer Customer { get; set; } public List Items { get; set; } } public class OrderItem { public int Id { get; set; } public int OrderId { get; set; } public string ProductName { get; set; } public int Quantity { get; set; } public double UnitPrice { get; set; } } } ///

/// Helper class to manage environment variables with fallback values ///

public class EnvironmentVariableManager { ///

/// Gets a value from environment variables with a fallback default ///

public string GetValue(string key, string defaultValue) { var value = Environment.GetEnvironmentVariable(key); return string.IsNullOrWhiteSpace(value) ? defaultValue : value; } } } // Extensions method for Dapper (to simulate Dapper behavior) public static class DapperExtensions { public static Task ExecuteAsync(this IDbConnection connection, string sql, object param = null) { // Simplified for the example using (var command = connection.CreateCommand()) { command.CommandText = sql; if (param is IEnumerable

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