在 linq 查询中执行左外连接

Question

我有下面的查询，它在带有左外连接的 sqllite 中工作

        select * from customer cu
        inner join contract cnt on cu.customerId = cnt.customerid
        inner join address addy on cu.addressid = addy.addressId
        inner join csrAssoc cassc on cu.customerid = cassc.customerId
        left outer join CustomerServiceRepresentative csrr  on cassc.csrid = csrr.customerservicerepresentativeId
        inner join customerServiceManager csmm on cassc.csmid = csmm.customerservicemanagerId
        where cu.customernumber = '22222234'

我希望能够在下面的 linq 查询中的这一行应用左外连接

        join csrr in objCsrCustServRep.AsEnumerable() on cassc.CsrId equals
                                    csrr.CustomerServiceRepresentativeId

        VisitRepData = (from cu in objCustomer.AsEnumerable()
                                join cnt in objContract.AsEnumerable() on cu.customerId equals cnt.customerId
                                join addy in objAddress.AsEnumerable() on cu.addressId equals addy.addressId
                                join cassc in objCsrAssoc.AsEnumerable() on cu.customerId equals cassc.CustomerId
                                join csrr in objCsrCustServRep.AsEnumerable() on cassc.CsrId equals
                                    csrr.CustomerServiceRepresentativeId
                                join csmm in objCustServMan on cassc.CsmId.ToString() equals csmm.customerServiceManagerId
                                where cu.CustomerNumber == (customernbr)

如何在 linq 查询中进行左外连接？

这是我调整后的评论和运行代码。还添加了其他部分。我得到的只是对象未设置为对象的实例。

         var VisitRepData = from cu in objCustomer.AsEnumerable()
                                join cnt in objContract.AsEnumerable() on cu.customerId equals cnt.customerId
                                join addy in objAddress.AsEnumerable() on cu.addressId equals addy.addressId
                                join cassc in objCsrAssoc.AsEnumerable() on cu.customerId equals cassc.CustomerId
                                join csrr in objCsrCustServRep.AsEnumerable() on cassc.CsrId equals
                                    csrr.CustomerServiceRepresentativeId into temp
                                from tempItem in temp.DefaultIfEmpty()
                                join csmm in objCustServMan on cassc.CsmId.ToString() equals csmm.customerServiceManagerId
                                where cu.CustomerNumber == (customernbr)
                                select new
                                {
                                    cu.customerId,
                                    cu.CustomerNumber,
                                    cu.customerName,
                                    cu.dateActive,
                                    cnt.contractExpirationDate,
                                    addy.street,
                                    addy.street2,
                                    addy.city,
                                    addy.state,
                                    addy.zipcode,
                                    cu.EMail,
                                    cu.phoneNo,
                                    cu.faxNumber,
                                    csmm.customerServiceManagerName,
                                    tempItem.CustomerServiceRepresentativeName,
                                };

                            foreach (var item in VisitRepData)
                            {
                                var one = item.customerId;
                                var two = item.CustomerNumber;
                            }

Answer 1

根据 documentation:

A left outer join is a join in which each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection. You can use LINQ to perform a left outer join by calling the DefaultIfEmpty method on the results of a group join

（强调我的）

基于上面的 MSDN link，如果我正确理解了您的要求，查询应该如下所示：

    VisitRepData = from cu in objCustomer.AsEnumerable()
                            join cnt in objContract.AsEnumerable() on cu.customerId equals cnt.customerId
                            join addy in objAddress.AsEnumerable() on cu.addressId equals addy.addressId
                            join cassc in objCsrAssoc.AsEnumerable() on cu.customerId equals cassc.CustomerId
                            join csrr in objCsrCustServRep.AsEnumerable() on cassc.CsrId equals
                                csrr.CustomerServiceRepresentativeId into temp
                            from tempItem in temp.DefaultIfEmpty()
                            join csmm in objCustServMan on cassc.CsmId.ToString() equals csmm.customerServiceManagerId
                            where cu.CustomerNumber == (customernbr)

具体的左外连接是用这段代码进行的：

join csrr in objCsrCustServRep.AsEnumerable()
    on cassc.CsrId equals csrr.CustomerServiceRepresentativeId 
    into temp
from tempItem in temp.DefaultIfEmpty()

在 linq 查询中执行左外连接

Doing a Left outer join in a linq query

linq

linq-to-sql