C# 中的 LINQ to SQL 查询
Posted
技术标签:
【中文标题】C# 中的 LINQ to SQL 查询【英文标题】：LINQ to SQL query in C# 【发布时间】：2020-03-18 10:08:10 【问题描述】：我是 C# /LINQ 查询的初学者 - 我有以下查询。右外连接在简单 SQL 中工作正常，但是，我无法在 LINQ 中实现它，但没有得到任何结果。
SELECT bt.PosGUID,s.PosGUID
FROM tblSchedule s right Join tblTrade bt
ON s.PosGUID = bt.PosGUID AND RowType = '4'
SELECT bt.PosGUID,s.PosGUID
FROM tblTrade bt left Join tblSchedule s
ON s.PosGUID = bt.PosGUID AND RowType = '4'
我需要了解上述左外连接的最佳方式是什么，我猜右外连接是不可能的，因此转换为左连接并尝试实现。
类似的东西 - 似乎有点复杂的查询：
var tQuery = from bt in m_DataContext.tblTrades
join bPos in m_DataContext.tblBigPositionStatics on
new bt.PosGUID equals
new bPos.PosGUID
join bo in m_DataContext.tblBigOrders
on new bt.ClOrdID, bt.PosGUID equals new bo.ClOrdID, bo.PosGUID
join tradingAcc in m_DataContext.tblTradingAccounts
on new Entity = bPos.PosEntity, Account = bPos.PosAccount equals
new tradingAcc.Entity, tradingAcc.Account
join btRef in m_DataContext.tblTrades.DefaultIfEmpty()
on new bt.PosGUID, ExecID = bt.ExecRefID equals new btRef.PosGUID, btRef.ExecID
into temp
from btref in temp.DefaultIfEmpty()
join desk in m_DataContext.tblDesks
on bt.PosDeskGUID equals desk.GUID
// JOIN not working not briging back all records from TBLTrades
join ss in m_DataContext.tblSchedules on bt.PosGUID equals ss.PosGUID into temp1
from ss in temp1.DefaultIfEmpty()
where bt.CreateDateTime >= dateToRun.getDate(false)
&& bt.CreateDateTime < dateToRun.getDate(false).AddDays(1)
&& bo.AsOfDateTime.Date == bt.AsOfDateTime.Date
&& bPos.HardDeleteDate == null
&& ss.RowType == "4"
//&& !"1".Equals(bt.ExecTransType)
//&& bt.HasBeenCorrected == false
&& deskGuidList.Contains(desk.GUID)
select new bt, bo, desk, bPos, tradingAcc, btref,ss ;
【问题讨论】：
您在 C# 中使用什么在 SQL 中进行查询？实体框架？英孚核心？其他 ？还请显示所涉及的数据类的最小定义。 旧的 LinqToSql 技术好吗？好用，但过时了；） 你可以不将两列命名相同，如果你这样做，你怎么知道哪一列有什么数据？您的 SQL 应该类似于SELECT bt.PosGUID AS bt_PosGUID,s.PosGUID AS s_PosGUID
这能回答你的问题吗？ LINQ to SQL Left Outer Join
谢谢 - 为我的原始查询添加了更多详细信息。
【参考方案1】：
如果要对表 A 和 B 进行右外连接，只需交换这些表即可进行左外连接。
左外连接是一个 GroupJoin，后跟一个 SelectMany。根据我的经验，我使用 GroupJoin 的频率远高于使用 Left Outer Join 的频率。尤其是在一对多关系中。
例如：假设您有一张 Schools 表和一张 Student 表。每个学校都有零个或多个学生，每个学生都在一个学校学习，使用外键 SchoolId：一个简单的一对多关系。
给我所有学校及其所有学生
var result = dbContext.Schools
.GroupJoin(dbContext.Students, // GroupJoin Schools and Students
school => school.Id, // from every School take the primary key
student => student.SchoolId, // from every Student take the foreign key
(school, studentsOnThisSchool) => new // from every School with all its Students
// make one new object
// Select only the School properties I plan to use
Id = schoolId,
Name = school.Name,
OlderStudents = studentsOnThisSchool
.Select(student => new
// Select only the Student properties I plan to use:
Id = student.Id,
Name = student.Name,
...
// not needed, I already know the value:
// SchoolId = student.SchoolId,
);
结果将是这样的序列：
School 1 with Students A, B, C, D.
School 2 with Students E, F,
School 3 without any Students
School 4 with Students G, H, I,
...
在我看来，这比左外连接的结果有用得多：
School 1 with Student A,
School 2 with Student E,
School 3 with Null student,
School 1 with Student B,
School 2 with Student F,
School 1 with Student C,
...
但是，嘿，这是你的选择。
我有一个 TblTrade，其中包含交易。每个 Trade 至少具有 Bt、PosGuid 和 RowType 属性。 我还有一个包含日程表的 TblSechedule。每个 Schedule 至少具有 Bt 和 PosGuid 属性。 向我提供所有具有相同 PosGuid 值的零个或多个 Schedules 的 RowType 4 交易。
var result = tblTrade
// keep only the trades that have RowType equal to 4:
.Where(trade => trade.RowType == 4)
// do the GroupJoin:
.GroupJoin(tblSchedule,
trade => trade.PosGuid,
schedule => schedule.PosGuid,
(trade, schedulesWithSamePosGuid) => new
// Select the trade properties you plan to use:
TradeId = trade.Id,
PosGuid = trade.PosGuid,
...
Schedules = schedulesWithSamePosGuid.Select(schedule => new
// Select the schedule properties you plan to use:
Id = schedule.Id,
...
// not needed, you already know the value:
// PosGuid = schedule.PosGuid.
)
.ToList(),
);
如果你真的想要一个扁平的左外连接，添加一个 SelectMany：
.SelectMany(groupJoinResult.Schedules,
(trade, schedule) => new
PosGuid = trade.PosGuid,
// the Trade properties:
Trade = new
Id = trade.TradeId,
...
,
Schedule = new
Id = schedule.Id,
...
,
);
如果你愿意，你可以创建一个扩展函数LeftOuterJoin：
public static class MyQueryableExtensions
// version without EqualityComparer:
public static IQueryable<TResult> LeftOuterJoin<T1, T2, TKey, TResult>(
this IQueryable<T1> source1,
IQueryable<T2> source2,
Func<T1, TKey> key1Selector,
Func<T2, TKey> key2Selector,
Func<T1, T2, TResult) resultSelector)
return LeftOuterJoin(source1, source2,
key1Selector, key2Selector, resultSelector, null);
具有 EqualityComparer 的版本：
public static IQueryable<TResult> LeftOuterJoin<T1, T2, TKey, TResult>(
this IQueryable<T1> source1,
IQueryable<T2> source2,
Func<T1, TKey> key1Selector,
Func<T2, TKey> key2Selector,
Func<T1, T2, TResult) resultSelector,
IEqualityComparer<TKey> comparer)
if (comparer == null) comparer = EqualityComparer<TKey>.Default;
// GroupJoin followed by SelectMany:
return GroupJoin(source1, source2, key1Selector, key2Selector,
(source1Item1, source2ItemsWithSameKey) => new
Source1Item = source1Item,
Source2Items = source2ItemsWithSameKey,
)
.SelectMany(groupJoinResult => groupJoinResult.Source2Items,
(groupJoinResult, source2Item) =>
ResultSelector(groupJoinResult.Source1Item, source2Item));
用法：
var result = tblTrade
.Where(trade => trade.RowType == 4)
.LeftOuterJoin(tblSchedule,
trade => trade.PosGuid,
schedule => schedule.PosGuid,
(trade, schedule) => new
// Select the trade and Schedule properties that you plan to use
// for example the complete trade and schedule:
Trade = trade,
Schedule = schedule,
// or only some properties:
CommonPosGuid = trade.PosGuid,
Trade = new
Id = trade.Id,
...
Schedule = new
Id = trade.Id,
...
)
【讨论】：
非常感谢您的解释 - 我会试一试，如果这有帮助，请告诉您。以上是关于C# 中的 LINQ to SQL 查询的主要内容，如果未能解决你的问题，请参考以下文章