DefaultIfEmpty
时间: 2023-11-08 09:56:25 浏览: 34
DefaultIfEmpty is a LINQ extension method that returns a new sequence with a default value if the original sequence is empty. It takes one parameter, which is the default value to return if the original sequence is empty.
For example, consider the following LINQ query:
```
int[] numbers = { };
var result = numbers.DefaultIfEmpty(0);
```
In this query, the `DefaultIfEmpty` method is used to return a new sequence with a default value of 0 because the `numbers` sequence is empty. The resulting `result` sequence will contain a single element with the value of 0. If the `numbers` sequence was not empty, the resulting `result` sequence would be the same as the original `numbers` sequence.
相关问题
join b in _materialRepository on a.MaterialId equals b.Id into bTemp from bLeftJoin in bTemp.DefaultIfEmpty()
这是 C# 中使用 LINQ 进行左连接查询的语句。
其中,`_materialRepository` 是一个数据仓储(Repository),`a.MaterialId` 是当前查询对象的属性,`b.Id` 是数据仓储中的属性。`into bTemp` 是将左连接查询的结果赋值给一个临时变量 `bTemp`,`from bLeftJoin in bTemp.DefaultIfEmpty()` 是将 `bTemp` 中的结果进行空值处理后赋值给 `bLeftJoin`。
这段代码的作用是:在当前查询对象中,根据 `MaterialId` 属性与数据仓储 `_materialRepository` 中的 `Id` 属性进行左连接查询,将查询结果赋值给 `bLeftJoin` 变量。如果查询结果为空,则将 `bLeftJoin` 变量赋值为默认值(例如 `null`)。
将以下linq代码转为sql server var query = from a in _AppointmentInfoDAL.Table.Where(a => a.Code == appointmentCode) join ad in _AppointmentDetailDAL.Table.Where(a => hisKeys.Contains(a.HisKey)) on a.Code equals ad.MasterCode join d in _InspectionItemDetailDAL.Table on ad.ItemCode equals d.Code join h in _HisBillInfoDAL.Table.Where(a => a.Status == 0) on ad.HisKey equals h.ItemId join pn in _PartNameDAL.Table on ad.PartCode equals pn.Code into pn2 from pn3 in pn2.DefaultIfEmpty() from i3 in _InspectionItemNoteDAL.Table.Where(p => p.InspItemCode == a.InspectionCode && (p.Type == 0 || p.Type.ToString() == a.PatientType)).DefaultIfEmpty() join p in _PatientInfoDAL.Table on a.PatientCode equals p.Code join e in _ExamineRoomInfoDAL.Table on a.RoomCode equals e.Code join it in _InspectionItemDAL.Table on d.InspItemCode equals it.Code join di in _DepartmentInfoDAL.FindList() on e.DeptCode equals di.Code join ph in _PhysicalRoomInfoDAL.Table on a.PhysicalRoomCode equals ph.Code into ph2 from ph3 in ph2.DefaultIfEmpty()
SELECT *
FROM AppointmentInfo a
INNER JOIN AppointmentDetail ad ON a.Code = ad.MasterCode
INNER JOIN InspectionItemDetail d ON ad.ItemCode = d.Code
INNER JOIN HisBillInfo h ON ad.HisKey = h.ItemId
LEFT OUTER JOIN PartName pn ON ad.PartCode = pn.Code
LEFT OUTER JOIN InspectionItemNote i3 ON i3.InspItemCode = a.InspectionCode AND (i3.Type = 0 OR CAST(i3.Type AS varchar) = a.PatientType)
INNER JOIN PatientInfo p ON a.PatientCode = p.Code
INNER JOIN ExamineRoomInfo e ON a.RoomCode = e.Code
INNER JOIN InspectionItem it ON d.InspItemCode = it.Code
INNER JOIN DepartmentInfo di ON e.DeptCode = di.Code
LEFT OUTER JOIN PhysicalRoomInfo ph3 ON a.PhysicalRoomCode = ph3.Code
WHERE a.Code = appointmentCode AND hisKeys.Contains(ad.HisKey) AND h.Status = 0
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