LINQ查询多个表？

Question

我正在使用LINQ加入这两个表来匹配模型类，如下所示

    lstOptInInterest = new LinkedList<OptInInterestArea>
                       ((from a in dbEntities.SUBCODE
                       from appCode in dbEntities.CODE.Where(
                          x => x.CODE == a.CODE  && x.TYPE == a.TYPE)
                        select new OptInInterestArea()
                        {
                               Code = a.CODE,
                               SubCode = a.SUBCODE,
                               SubCodeDescription = a.DESCR,
                               CodeDescription = appCode.DESCR
                          }).ToList());

模型类

public class OptInInterestArea
{      
        [DisplayName("Code")]
        public string Code { get; set; }

        [DisplayName("Sub Code")]
        public string SubCode { get; set; }

        DisplayName("Sub Code Description")]
        public string SubCodeDescription { get; set; }

        [DisplayName("Code Description")]
        public string CodeDescription { get; set; }

        [DisplayName("Previous OptIn")]
        public bool PrevOptIn { get; set; }
}

表B

现在我的问题是我需要从表B中分配PrevOptIn lstOptInInterest的值（见上文）。表B可能包含也可能不包含CODE的所有SUBCODE和lstOptInInterest。

如果表格B中存在CODE和SUBCODE lstOptInInterest，则分配给PrevOptIn中的PrevOptIn = N

我如何做LINQ才能得到这个？

Answer 1

首先，我建议您使用INNER JOIN代替CROSS JOIN来查询您的lstOptInInterest：

lstOptInInterest = new LinkedList<OptInInterestArea>
                       ((from a in dbEntities.SUBCODE
                         join appCode in dbEntities.CODE
                         on new {CODE=a.CODE,TYPE=a.TYPE} equals new {CODE=x.CODE,TYPE=x.TYPE}
                         select new OptInInterestArea()
                          {
                               Code = a.CODE,
                               SubCode = a.SUBCODE,
                               SubCodeDescription = a.DESCR,
                               CodeDescription = appCode.DESCR
                          }).ToList());

seconde，使用LEFT OUT JOIN从表B中分配lstOptInInterest的PrevOptIn值：

lstOptInInterest = new LinkedList<OptInInterestArea>
            (
                  (from a in lstOptInInterest
                          join b in dbEntities.TableB
                          on new {CODE=a.Code,SUBCODE=a.SubCode} equals new {CODE=b.CODE,SUBCODE=b.SUBCODE}
                          into leftGroup
                          from b in leftGroup.DefaultIfEmpty()
                          select new OptInInterestArea()
                          {
                               Code = a.Code,
                               SubCode = a.SubCode,
                               SubCodeDescription = a.SubCodeDescription,
                               CodeDescription = a.CodeDescription,
                               PrevOptIn=b==null? false : b.OPTIN=="Y"
                          }).ToList()
            );

<强>更新 尝试只测试一步以获得结果：

lstOptInInterest = new LinkedList<OptInInterestArea>
                       ((from a in dbEntities.SUBCODE
                         join appCode in dbEntities.CODE
                         on new {CODE=a.CODE,TYPE=a.TYPE} equals new {CODE=x.CODE,TYPE=x.TYPE}
                         join b in dbEntities.TableB
                         on new {CODE=a.CODE,SUBCODE=a.SUBCODE} equals new {CODE=b.CODE,SUBCODE=b.SUBCODE}
                         into leftGroup
                         from b in leftGroup.DefaultIfEmpty()
                         select new OptInInterestArea()
                          {
                               Code = a.CODE,
                               SubCode = a.SUBCODE,
                               SubCodeDescription = a.DESCR,
                               CodeDescription = appCode.DESCR,
                               PrevOptIn=b==null?false : b.OPTIN=="Y"
                          }).ToList());