How to join 3 tables with lambda expression?

问题:

I have a simple LINQ lambda join query but I want to add a 3rd join with a where clause. How do I go about doing that?

Here's my single join query:

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
    });

I want to add the following SQL command to the above LINQ query and still maintain the projections:

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

The 3rd join would be made with Sector table & Distribution_sector_industry on sector_code.

Thanks in advance.

回答1:

Just a guess:

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
});

回答2:

Okay, I can't see why you'd want to select sector_code when you already know it, but I think you want this:

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
            };

Notes:

• I've changed it into a query expression as that's a much more readable way of expressing a query like this.
• Although the "where" clause comes after the join, assuming this is a LINQ to SQL or Entity Framework query, it shouldn't make any difference
• I've lengthened the range variable names for clarity
• I've converted your other names into conventional .NET names; you can do this too in your model

回答3:

Try something like this...

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 {...});

回答4:

For 4 Tables

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
});