Question

我在下面的查询在sqllite中使用左外连接

        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查询中对此行应用左外连接

        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查询中执行左外连接？

调整并运行代码后，这是我的评论。另一部分也加入了。所有我得到的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

每documentation：

左外连接是一个连接，其中返回第一个集合的每个元素，无论它是否在第二个集合中有任何相关元素。 您可以使用LINQ通过对组连接的结果调用DefaultIfEmpty方法来执行左外连接

（强调我的）

基于上面的MSDN链接，如果我正确理解您的要求，查询应如下所示：

    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)

具体来说，左外连接是使用以下代码执行的：

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

1 个答案: