C#クエリ式で効率的なデータアクセスを実現する方法

var results = from item in collection
              where item.Property == someValue
              select item;

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var evenNumbers = from number in numbers
                  where number % 2 == 0
                  select number;

foreach (var num in evenNumbers)
{
    Console.WriteLine(num);
}

var evenNumbers = numbers.Where(n => n % 2 == 0).Select(n => n);

using (var context = new MyDbContext())
{
    var results = from user in context.Users
                  where user.Age > 18
                  select user;

    foreach (var user in results)
    {
        Console.WriteLine($"{user.Name} - {user.Age}");
    }
}

var employees = new List<Employee>
{
    new Employee { Name = "Alice", Department = "HR", Salary = 50000 },
    new Employee { Name = "Bob", Department = "IT", Salary = 60000 },
    new Employee { Name = "Charlie", Department = "HR", Salary = 70000 },
    new Employee { Name = "David", Department = "IT", Salary = 80000 }
};

var groupedEmployees = from emp in employees
                       group emp by emp.Department into deptGroup
                       select new
                       {
                           Department = deptGroup.Key,
                           AverageSalary = deptGroup.Average(e => e.Salary)
                       };

foreach (var group in groupedEmployees)
{
    Console.WriteLine($"{group.Department} - Average Salary: {group.AverageSalary}");
}

XDocument xdoc = XDocument.Load("employees.xml");

var employeeNames = from employee in xdoc.Descendants("Employee")
                    where (int)employee.Element("Age") > 30
                    select employee.Element("Name").Value;

foreach (var name in employeeNames)
{
    Console.WriteLine(name);
}

var query = from item in collection
            where item.Property == someValue
            select item;

// 遅延実行
var results = query.ToList();

// 即時実行
var immediateResults = query.ToList();

var optimizedQuery = from item in collection
                     where item.Property == someValue
                     select new { item.Property, item.OtherProperty };

var selectedColumns = from item in context.Items
                      select new { item.Id, item.Name };

if (collection != null)
{
    var results = from item in collection
                  where item.Property != null
                  select item;
}

if (collection.Any())
{
    var firstItem = collection.First();
}
else
{
    // エラーハンドリングコード
}

try
{
    using (var context = new MyDbContext())
    {
        var results = from user in context.Users
                      where user.Age > 18
                      select user;

        foreach (var user in results)
        {
            Console.WriteLine($"{user.Name} - {user.Age}");
        }
    }
}
catch (SqlException ex)
{
    Console.WriteLine("データベース接続エラー: " + ex.Message);
    // ログの記録やリトライ処理など
}

try
{
    // クエリ実行コード
}
catch (Exception ex)
{
    Console.WriteLine("エラーが発生しました。詳細は管理者にお問い合わせください。");
    // 詳細エラーはログに記録
    LogError(ex);
}

using (var context = new MyDbContext())
{
    string userInput = "someValue";
    var results = from user in context.Users
                  where user.Name == userInput
                  select user;

    foreach (var user in results)
    {
        Console.WriteLine(user.Name);
    }
}

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string EncryptedPassword { get; set; }
}

// 保存時
var user = new User
{
    Name = "John Doe",
    EncryptedPassword = Encrypt("password123")
};

// 復号化
string decryptedPassword = Decrypt(user.EncryptedPassword);

CREATE USER limited_user WITH PASSWORD 'password';
GRANT SELECT, INSERT, UPDATE ON my_table TO limited_user;

public void LogQuery(string query)
{
    // クエリのログを記録
    File.AppendAllText("query.log", query);
}

List<Person> people = new List<Person>
{
    new Person { Name = "Alice", Age = 25 },
    new Person { Name = "Bob", Age = 30 },
    new Person { Name = "Charlie", Age = 35 },
    new Person { Name = "David", Age = 40 }
};

var result = from person in people
             where person.Age >= 30
             select person.Name;

foreach (var name in result)
{
    Console.WriteLine(name);
}

List<Employee> employees = new List<Employee>
{
    new Employee { Name = "Alice", Department = "HR", Salary = 50000 },
    new Employee { Name = "Bob", Department = "IT", Salary = 60000 },
    new Employee { Name = "Charlie", Department = "HR", Salary = 70000 },
    new Employee { Name = "David", Department = "IT", Salary = 80000 }
};

var result = from employee in employees
             where employee.Department == "IT" && employee.Salary >= 60000
             select employee.Name;

foreach (var name in result)
{
    Console.WriteLine(name);
}

List<Employee> employees = new List<Employee>
{
    new Employee { Name = "Alice", Department = "HR", Salary = 50000 },
    new Employee { Name = "Bob", Department = "IT", Salary = 60000 },
    new Employee { Name = "Charlie", Department = "HR", Salary = 70000 },
    new Employee { Name = "David", Department = "IT", Salary = 80000 }
};

var groupedEmployees = from emp in employees
                       group emp by emp.Department into deptGroup
                       select new
                       {
                           Department = deptGroup.Key,
                           AverageSalary = deptGroup.Average(e => e.Salary)
                       };

foreach (var group in groupedEmployees)
{
    Console.WriteLine($"{group.Department} - Average Salary: {group.AverageSalary}");
}

XDocument xdoc = XDocument.Load("employees.xml");

var employeeNames = from employee in xdoc.Descendants("Employee")
                    where (int)employee.Element("Age") >= 30
                    select employee.Element("Name").Value;

foreach (var name in employeeNames)
{
    Console.WriteLine(name);
}