LINQ有条件地使用不同的字段来加入

Question

I have this situation:

Table A 
Field A  Field B
   d       1 
   d       2
   d       3
Table B 
Field A  Field B
   d       1
   d       2
   d       3
   d       4

I want the "4" in Table B to match to the "3" in Table A.
Here's my code currently:

short maxFieldB = DatabaseContext.TableA
    .OrderByDescending(x => x.FieldB)
    .Select(s => s.FieldB)
    .First();

IQueryable<FieldXType> fieldX = (from tableb in DatabaseContext.TableB
    join tablea in DatabaseContext.TableA on new 
    {
        a = tablea.FieldA,
        b = tablea.FieldB
    }
    equals new
    {
        a = tableb.FieldA,
        b = (tableb.FieldB > maxFieldB ? maxFieldB : tableb.FieldB)
    })
    .ToList();

When I run this, I get an exception "Argument types do not match". The stack trace seems to point at my statement that includes the ternary operator.

(at System.Linq.Expressions.Expression.Condition(Expression test, 
Expression ifTrue, Expression ifFalse))  

I thought I was doing the equivalent to a T-SQL CASE statement. Am I going about this wrong? If so, how do I make the "4" on "Table B" match to the "3" on "Table A"? Does it have anything to do with my switching from Lambda syntax to query syntax in the middle?

Thanks in advance.

Answer 1

Alright, as far as i understood your question, You have two tables and you want to join them on multiple conditions. The error is correct "Argument types do not match". Here are few issues with the code:

IQueryable<FieldXType> fieldX = (from tableb in DatabaseContext.TableB
        join tablea in DatabaseContext.TableA on new Eaxmple1 //add a type name that contains defination for a and b
        {
            a = tablea.FieldA,
            b = tablea.FieldB//This is of some type say FieldB(as defined in your Model Eaxmple1)
        }
        equals new Eaxmple1 
        {
            a = tableb.FieldA,
            b = (tableb.FieldB > maxFieldB ? maxFieldB : tableb.FieldB) //and this maxFieldB is of another type 'short' as you defined above.
        } select tablea).ToList();

Also your query is missing select statement which cases the issue.

Here are some quick fixes that might be helpful:

Create a new model that contains the definition for a and b if not created already and make sure that their type should same as that of your table. eg

public class Eaxmple1
{
public field a {get;set;}
public field b {get;set;}
}

I can help better if you can provide the more details of the tables.