如何使用lambda表达式连接3个表？

我有一个简单的LINQ lambda连接查询，但我想添加一个带有where子句的第3个连接。 我该怎么做呢？

这是我的单个连接查询：

var myList = Companies .Join( Sectors, comp => comp.Sector_code, sect => sect.Sector_code, (comp, sect) => new {Company = comp, Sector = sect} ) .Select( c => new { c.Company.Equity_cusip, c.Company.Company_name, c.Company.Primary_exchange, c.Company.Sector_code, c.Sector.Description }); 

我想将以下SQL命令添加到上面的LINQ查询中并仍然保持预测：

 SELECT sector_code, industry_code FROM distribution_sector_industry WHERE service = 'numerical' 

第3次连接将使用Sector_code上的Sector表和Distribution_sector_industry进行。

提前致谢。

Answer 1

只是一个猜测：

 var myList = Companies .Join( Sectors, comp => comp.Sector_code, sect => sect.Sector_code, (comp, sect) => new { Company = comp, Sector = sect }) .Join( DistributionSectorIndustry.Where(dsi => dsi.Service == "numerical"), cs => cs.Sector.Sector_code, dsi => dsi.Sector_code, (cs, dsi) => new { cs.Company, cs.Sector, IndustryCode = dsi.Industry_code }) .Select(c => new { c.Company.Equity_cusip, c.Company.Company_name, c.Company.Primary_exchange, c.Company.Sector_code, c.Sector.Description, c.IndustryCode }); 

Answer 2

好的，我不明白你为什么要在你已经知道的时候选择sector_code，但我想你想要这个：

 var query = from company in Companies join sector in Sectors on company.SectorCode equals sector.SectorCode join industry in DistributionSectorIndustry on sector.SectorCode equals industry.SectorCode where industry.Service == "numerical" select new { company.EquityCusip, company.CompanyName, company.PrimaryExchange, company.SectorCode, sector.Description, industry.IndustryCode }; 

笔记：

• 我已将其更改为查询表达式，因为这是一种表达此类查询的可读方式。
• 虽然“where”子句在连接之后出现，但假设这是一个LINQ to SQL或Entity Framework查询，它应该没有任何区别
• 为了清晰起见，我已经延长了范围变量名称
• 我已将您的其他名称转换为传统的.NET名称; 您也可以在模型中执行此操作

Answer 3

试试这样的事……

 var myList = ({from a in Companies join b in Sectors on a.Sector_code equals b.Sector_code join c in Distribution on b.distribution_code equals a.distribution_code select new {...}); 

Answer 4

4桌

 var query = CurrencyDeposits .Join(Customers, cd => cd.CustomerId, cus => cus.Id, (cd, cus) => new { CurrencyDeposit = cd, Customer = cus }) .Join(Currencies, x => x.CurrencyDeposit.CurrencyId, cr => cr.Id, (x, cr) => new { x.CurrencyDeposit, x.Customer, Currency = cr }) .Join(Banks, x => x.CurrencyDeposit.BankId, bn => bn.Id, (x, bn) => new { x.CurrencyDeposit, x.Customer, x.Currency, Bank = bn}) .Select(s => new { s.CurrencyDeposit.Id, s.Customer.NameSurname, s.Currency.Code, s.Bank.BankName, s.CurrencyDeposit.RequesCode });