LINQ - 使用 group by 和 sum 加入 3 个表答案

【问题标题】：LINQ - Join 3 tables with group by and sumLINQ - 使用 group by 和 sum 加入 3 个表
【发布时间】：2016-08-12 03:50:44
【问题描述】：

我尝试将以下 SQL 查询转换为 LINQ：

Select Employee.Name,
       ts.HoursWorked,
       People.Date 
FROM [dbo].[HoursWorked] as Employee
JOIN (SELECT [Employee], Sum([LaborTime]) as HoursWorked
      FROM [dbo].[TimeSheet]
      GROUP BY [Employee] ) ts
ON Employee.Employee = ts.Employee
JOIN [dbo].[PeopleData] People
ON Employee.Employee = People.Employee

我尝试了以下方法，但没有返回预期结果：

from ts in _context.TimeSheet
join es in _context.HoursWorked on ts.Employee equals es.Employee
join ed in _context.EmployeeDetailed on ts.Employee equals ed.Employee
group ts by ts.Employee into g
select new 
{
    Name = g.Key,
    HoursWorked = g.Sum(e => e.LaborTime),
    FirstDate = ??? //Not sure how to access es
};

【问题讨论】：

标签： c# linq join linq-to-sql group-by

【解决方案1】：

尝试以下查询。我已经更改了joins 的顺序，只是为了让它像您的 sql 查询一样。您在尝试中缺少的部分不仅仅是按名称进行分组，因为现在它不在嵌套查询中（如果您尝试在没有嵌套查询的情况下执行 sql，您也必须这样做）

var result = from es in _context.HoursWorked
             join ts in _context.TimeSheet on es.Employee equals ts.Employee
             join ed in _context.EmployeeDetailed on es.Employee equals ed.Employee
             group ts by new { es.Name, ed.Date } into g
             select new
             {
                 Name = g.Key.Name,
                 FirstDate = g.Key.Date,
                 HoursWorked = g.Sum(e => e.LaborTime),
             };

如果你想像你的 sql 一样创建查询，那么：

var result = from es in _context.HoursWorked
             join ts in _context.TimeSheet.GroupBy(item => item.Employee)
                                 .Select(g => new { Employee = g.Key, HourseWorked = g.Sum(item => item.LaborTime) })
             on es.Employee equals ts.Employee
             join ed in _context.EmployeeDetailed on es.Employee equals ed.Employee
             select new
             {
                 Name = es.Employee,
                 HoursWorked = ts.HourseWorked,
                 FirstDate = ed.Date
             };

在查询语法中：

var result = from es in _context.HoursWorked
             join ts in (from item in _context.TimeSheet
                         group item by item.Employee into g
                         select new { Employee = g.Key, HourseWorked = g.Sum(item => item.LaborTime) })
             on es.Employee equals ts.Employee
             join ed in _context.EmployeeDetailed on es.Employee equals ed.Employee
             select new
             {
                 Name = es.Employee,
                 HoursWorked = ts.HourseWorked,
                 FirstDate = ed.Date
             };

【讨论】：

【解决方案2】：

您的连接子句之一是子查询。为了使事情变得更容易，您应该将其分开：

var sub = from ts in _context.TimeSheet
          group ts by ts.Employee into g
          select new {
              Employee = g.Key,
              HoursWorked = g.Sum(p=>p.LaborTime)
          };

然后你可以像这样进行整个查询：

from emp in _context.HoursWorked
join s in sub on emp.Employee equals s.Employee
join pd in _context.PeopleData on emp.Employee equals pd.Employee
select new {emp.Name, s.HoursWorked, emp.FirstDate}

【讨论】：

【解决方案3】：

linq 查询的任何局部变量在组范围变量之后都无法实现。所以你不能访问 ed (ed.Date) 变量，因为它是在之前声明的

group ts by ts.Employee into g

此代码将起作用：

var result = ts in _context.TimeSheet
         group ts by ts.Employee into g
         let singleG = g.FirstOrDefault()
         join es in _context.HoursWorked  on singleG.Employee  equals  es.Employee
         join ed in _context.PeopleData on es.Employee equals ed.Employee 
         select new 
         {
          Name = g.Key,
          HoursWorked = g.Sum(e => e.LaborTime),
          FirstDate = ed.Date 
         };

【讨论】：