如何使用按组数据创建左连接

Question

我有三个表一是“Allowance”，“Balance”和“TimeoffRequests”这三个表中的常见列是EmployeeId和TimeoffTypeId ，现在我需要通过将表格“timeoffTypeId”中的EmployeeId和TimeoffRequests分组来获取一个请假时间，并获得“TimeOffHours”。因为我写了像

这样的代码

var query = (from tr in TimeOffRequests
             where tr.EmployeeID == 9
             group tr by new { tr.EmployeeID, tr.TimeOffTypeID } into res
             select new
             {
                 EmployeeID = res.Key.EmployeeID,
                 TimeOffTypeID = res.Key.TimeOffTypeID,
                 TotalHours = res.Sum(x => x.TimeOffHours)
             }).AsEnumerable();

现在我需要将这些结果与第一个表联系起来，并且必须得到所有员工，timeoffTypes和UserAllowance以及相应的 TimeoffHours 从分组表中。为了得到左连接查询我写如下。

var requestResult = (from UA in UserAllowances
                     join UB in UserBalances on UA.EmployeeID equals UB.EmployeeID
                     where UA.TimeOffTypeID == UB.TimeOffTypeID && UA.EmployeeID == 9
                      && UA.TimeOffType.IsDeductableType == true      // LeftJoin

                     join rest in query on UA.EmployeeID equals rest.EmployeeID into penidngRequst
                     from penReq in penidngRequst.DefaultIfEmpty()
                     where penReq.TimeOffTypeID == UA.TimeOffTypeID

                     select new EmployeeTimeOffBalanceModel
                     {
                         TimeOffTypeID = UA.TimeOffTypeID != null ? UA.TimeOffTypeID : 0,
                         YearlyAllowanceHrs = (UA.YearlyAllowanceHrs != null) ? UA.YearlyAllowanceHrs : 0,
                         BalanceHours = UB.BalanceHrs != null ? UB.BalanceHrs : 0,
                         PendingHours = (decimal)((penReq != null) ? (penReq.TotalHours) : 0),
                         EmployeeID = UA != null ? UA.EmployeeID : 0,


                     }).ToList().Distinct();

只有 timeOFfType 包含分组数据，即使我使用“into”和leftjoin关键字为查询撰写了DefaultIfEmpty()。结果如下：

并且通过使用“linqPad”编辑器，我发现它正在应用Cross或Outer Join而不是“左连接”，这将是原因。

如果我删除了这行代码 " where penReq.TimeOffTypeID == UA.TimeOffTypeID" ，则会显示timeoffTypes所有cross join并重复

如果timeofftypes没有任何请求，我怎样才能与包含Grouped数据的表实现左连接并显示空值？

Answer 1

您可能希望将where子句移动到on equals子句中，如下所示

join rest in query on new { UA.EmployeeID, UA.TimeOffTypeID } equals new { rest.EmployeeID, rest.TimeOffTypeID } into penidngRequst
from penReq in penidngRequst.DefaultIfEmpty()

Answer 2

您可以更改

where penReq.TimeOffTypeID == UA.TimeOffTypeID

到

where penReq == null || penReq.TimeOffTypeID == UA.TimeOffTypeID