如何使用lambda表达式连接3个表？

Question

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查询中并仍然保持投影:

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:

• 因为这是一个我已经改变它变成一个查询表达式多表达这样的查询的更易读的方式.
• 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
• 为了清楚起见,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; 你也可以在你的模型中做到这一点

Answer 3

4桌用

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
     });