Question

有许多地方展示了如何使用LINQ将两个表连接起来，但是我在添加更多连接时遇到了麻烦（我有15个表）。我以为我的模式已经下降，但我收到了错误。顺便说一句，这是针对强类型DataSet的，尽管我并不怀疑这一点。虽然我实际上有更多的表要添加到连接中，但我试图让它最初使用较少的连接（星号是发生错误的地方）：

var results = 
    from e in DataSetHelper.ds.Employee
    join es in DataSetHelper.ds.EmployeeSkill on e.EmployeeId equals es.EmployeeId    into esGroup from esItem in esGroup.DefaultIfEmpty()
    join s in DataSetHelper.ds.Skill on **es.SkillId** equals s.SkillId                   into skillGroup from skillItem in skillGroup.DefaultIfEmpty()
    join er in DataSetHelper.ds.EmployeeRole on e.EmployeeId equals er.EmployeeId     into erGroup from erItem in erGroup.DefaultIfEmpty()
    join r in DataSetHelper.ds.Role on **er.RoleId** equals r.RoleId                      into rGroup from rItem in rGroup.DefaultIfEmpty()

我遇到两个错误（同样的事情，但在不同的连接上）。它们位于查询的第3行和第5行。

对于es.SkillId,，错误为The name 'es' does not exist in the current context.

对于er.RoleId,，错误为The name 'er' does not exist in the current context.

同样，我需要将此模式用于另外十个连接，所以我希望这种模式不会随着我的复杂性而增加。

Answer 1

试试这个

var results = DataSetHelper.ds.Employee
.Join(DataSetHelper.ds.EmployeeSkill, e => e.EmployeeId, es => es.EmployeeId, (e, es) => new { e, es })
.Join(DataSetHelper.ds.Skill, esGroup  => esGroup.es.SkillId, s => s.SkillId, (esGroup, s) => new { esGroup.e, esGroup.es, s })
.Join(....

我已经展示了直到第一个多对多的关系。对于一切都会有点长，但我希望你能管理它。

Answer 2

这很有效。我还必须考虑连接列上是否存在null的可能性。我使用了空条件运算符。不幸的是，使用内存中的数据集与使用SQL Server一样慢（我甚至没有完成所有连接或所有where子句测试）。好像我需要使用不同的方法。

var results = from e in _data.ds.Employee
              join es in _data.ds.EmployeeSkill on e.EmployeeId equals es.EmployeeId        into esGroup from esItem in esGroup.DefaultIfEmpty()
              join s in _data.ds.Skill on esItem?.SkillId equals s.SkillId                  into sGroup from skillItem in sGroup.DefaultIfEmpty()
              join er in _data.ds.EmployeeRole on e.EmployeeId equals er.EmployeeId         into erGroup from erItem in erGroup.DefaultIfEmpty()
              join r in _data.ds.Role on erItem?.RoleId equals r.RoleId                     into rGroup from rItem in rGroup.DefaultIfEmpty()
              join et in _data.ds.EmployeeTechnology on e.EmployeeId equals et.EmployeeId   into etGroup from etItem in etGroup.DefaultIfEmpty()
              join t in _data.ds.Technology on etItem?.TechnologyId equals t.TechnologyId   into tGroup from tItem in etGroup.DefaultIfEmpty()
              where
                e.FirstName.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0 ||
                e.LastName.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0 ||
                e.RMMarket.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0 ||
                !e.IsSummaryNull() && e.Summary.IndexOf(searchTerm, StringComparison.OrdinalIgnoreCase) >= 0
              select new SearchResult
              {
                  EmployeeId = e.EmployeeId,
                  Name = e.FirstName + " " + e.LastName,
                  Title = e.Title,
                  ImageUrl = e.IsImageUrlNull() ? string.Empty : e.ImageUrl,
                  Market = e.RMMarket,
                  Group = e.Group,
                  Summary = e.IsSummaryNull() ? string.Empty : e.Summary.Substring(1, e.Summary.Length < summaryLength ? e.Summary.Length - 1 : summaryLength),
                  AdUserName = e.AdUserName
              };

LINQ to DataSet多表左连接

2 个答案: