使用 lambda 表达式从 1 个表返回值，同时比较 2 个表的值

Question

我不知道应该在标题中更准确地说什么，但我会解释我的问题/问题。

我正在做一个 API 并编写一个方法来获取表中的所有数据。

例子：

Table1 = 部门
ID_部门
名称_部门

---

表 2 = 员工 Id_Employee 姓名_员工 ID_部门 ...

我正在尝试编写一个 lambda 表达式，通过我在程序中传递给 API 的 Name_Department 从员工那里获取所有数据。其中表 1 中的 ID_department 等于表 2 中的 ID_Department，但不知道 ID_department 本身。

为了更容易理解，这里有一个示例查询（由 head 组成，因为属性和表不是这些）。

        SELECT EMPLOYEE.ID_EMPLOYEE, EMPLOYEE.NAME_EMPLOYEE, (OTHER EMPLOYEE ATTRIBUTES), DEPARTMENT.NAME_DEPARTMENT
        FROM EMPLOYEE INNER JOIN DEPARTMENT ON 
        EMPLOYEE.ID_DEPARTMENT = DEPARTMENT.ID_DEPARTMENT
        WHERE DEPARTMENT.NAME_DEPARTMENT LIKE '%FINANCE%'

提前致谢

Answer 1

不知道为什么要在数据集上使用 lambda 表达式...通常如下所示更好...（顺便说一下，这称为 Linq To Entities 或 L2E，不需要 lambda 表达式）

using (DBContext ctx = new DBContext())
{
    ctx.Connection.Open();

    List<EMPLOYEE> employees = (from e in ctx.EMPLOYEE 
                                join d in ctx.Department on e.IdDepartment equals d.IdDepartment
                                where d.NameDepartment.Contains("FINANCE")
                                select e).ToList();

    ctx.Connection.Close();
}

Answer 2

我为您编写了一个 LinqPad 示例，它将展示其工作原理（LinqPad 是免费下载的，您只需将代码粘贴到查询窗口并将语言下拉菜单更改为“C# 程序”）

public class Department
{
    public int DepartmentId {get; set;}
    public string Name {get; set;}
    public List<Employee> Employees {get; set;}
    public Department()
    {
        Employees = new List<Employee>();
    }
}

public class Employee
{
    public int EmployeeId {get; set;}
    public string Name {get; set;}
    public Department Department {get; set;}
}

void Main()
{
    // set up replica data (much like any ORM would return)  
    List<Department> Departments = new List<Department>();
    List<Employee> iTEmployees = new List<Employee>();
    List<Employee> salesEmployees = new List<Employee>();

    Department iT = new Department(){ Name = "IT", DepartmentId = 1 };
    iTEmployees.Add(new Employee(){EmployeeId = 1, Name = "Jo",  Department = iT});
    iTEmployees.Add(new Employee(){EmployeeId = 2, Name = "Jim",  Department = iT });
    iTEmployees.Add(new Employee(){EmployeeId = 3, Name = "James",  Department = iT });
    iT.Employees = iTEmployees;
    Departments.Add(iT);

    Department sales = new Department(){ Name = "Sales", DepartmentId = 1 };
    salesEmployees.Add(new Employee(){EmployeeId = 1, Name = "Jo",  Department = sales});
    salesEmployees.Add(new Employee(){EmployeeId = 2, Name = "Jan",  Department = sales });
    salesEmployees.Add(new Employee(){EmployeeId = 3, Name = "Jane",  Department = sales });
    sales.Employees = salesEmployees;
    Departments.Add(sales);

    // query objects
    string searchParam = "IT";

    List<Employee> employees = /*dataContext.*/Departments.SelectMany(x=>x.Employees.Where(y=>y.Department.Name.ToLowerInvariant() == searchParam.ToLowerInvariant())).ToList();
    employees.Dump();
}

希望对你有帮助