C＃-从SQL到Linq-左外部联接/内部联接

Question

I got this in SQL:

with 3 Values: value1, value2, value3

SELECT A.a, A.b, E.c, B.d, A.e, A.f, A.g, D.h, D.i 

FROM Alpha as A 

INNER JOIN Beta as B ON A.b = B.k 

LEFT OUTER JOIN Charlie as C ON C.a = A.a  

LEFT OUTER JOIN Delta as D ON D.k = B.j  

INNER JOIN Echo as E ON A.a = E.a     

WHERE A.a = @value1 AND E.c = @value2 AND (A.a = @value3 OR @value3 = '') AND A.b = E.b

Alpha, Beta, Charlie, Delta and Echo have string a,b,c,d,e,f,g,h,i,j,k.

I've tried to convert into Linq, but I don't get the Syntax of join right. Can you show me how it should look? x)

public static List<value> GetSmthn(string value1, string value2, string value3)
{           

    return (
         from A in Alpha

         join B in Beta on A.b equals B.k

         join C in Charlie on A.a equals C.a 

         join D in Delta on B.j equals D.k 

         join E in Echo on E.a equals A.a

         where (A.a == value1 && E.c == value2 && (A.a == value3 || value3 == "") && A.b == E.b)

         select new value() { a = A.a, b = A.b, c = E.c, d = A.d, e = A.e, f = A.f, g = A.g, h = D.h i = D.i }

         ).ToList();
}

Thanks for help, maybe it's a bit confusing. Cause I changed the variable-names. Sorry for the bad english btw

Answer 1

According to MSDN :

To perform a left outer join in LINQ, use the DefaultIfEmpty method in combination with a group join to specify a default right-side element to produce if a left-side element has no matches. You can use null as the default value for any reference type, or you can specify a user-defined default type.

From what I can tell, you first join all the data via a GroupJoin This "correlates the elements of two sequences based on key equality and groups the results." Then, the following from specifies a default right-side element (by using DefaultIfEmpty() ) to produce if a left-side element has no matches. You will need to perform this for all of your left joins:

public static List<value> GetSmthn(string value1, string value2, string value3)
    {           

    return (
         from A in Alpha

         join B in Beta on A.b equals B.k

         join C in Charlie on A.a equals C.a into cgroup
         from C in cgroup.DefaultIfEmpty()

         join D in Delta on B.j equals D.k into dgroup
         from D in dgroup.DefaultIfEmpty()

         join E in Echo on E.a equals A.a

         where (A.a == value1 && E.c == value2 && (A.a == value3 || value3 == "") && A.b == E.b)

         select new value() { a = A.a, b = A.b, c = E.c, d = A.d, e = A.e, f = A.f, g = A.g, h = D.h i = D.i }

         ).ToList();
    }