我在构建Linq查询时做错了什么

Question

I am trying to write equivalent linq code for following query.

SELECT A.*  
FROM  
(  
    SELECT * FROM TableA   
    WHERE id = 100  
) a  
JOIN   
(  
    SELECT Name, MAX(AnotherId) AnotherId   
    FROM TableA   
    WHERE id = 100  
    GROUP BY Name   
) b  
on a.Name  = b.Name and a.AnotherId = b.AnotherId   

This is the linq

var Collection = from R in DbContext.TableA  
join G in (DbContext.TableA.Where(r => r.Id == 100).GroupBy(r => new { r.Name, r.AnotherId } ).Select(g => new { Name = g.Name , AnotherId = g.Max(o => o.AnotherId) }))  
on new { R.Name, R.AnotherId } equals new { G.Name, G.AnotherId }  
where R.Id == 100  
select R;  

But I am getting following compilation error that I don't know how to fix. Any thoughts

The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'Join'.

Error 7 'System.Linq.IGrouping' does not contain a definition for 'Name' and no extension method 'Name' accepting a first argument of type 'System.Linq.IGrouping' could be found (are you missing a using directive or an assembly reference?)

Answer 1

You group by r.Name, r.AnotherId, when you just want to group by r.Name.

var Collection = from R in DbContext.TableA  
join G in (DbContext.TableA
                      .Where(r => r.Id == 100)
                      .GroupBy(r => r.Name)
                      .Select(g => new { Name = g.Key , AnotherId = g.Max(o => o.AnotherId) }))  
on new { R.Name, R.AnotherId } equals new { G.Name, G.AnotherId }  
where R.Id == 100  
select R; 

And to have all in Fluent Syntax

var collection = DbContext.TableA
                          .Where(t1 => t1.Id == 100)
                          .Join(DbContext.TableA
                                .Where(t2 => t2.Id == 100)
                                .GroupBy(t2 => t2.Name)
                                .Select(group => new{Name = group.Key, 
                                                      AnotherId = group.Max(e => e.AnotherId)})
                                 ),
                                 t1 => new{t1.Name, t1.AnotherId} ,
                                 t2 => new{t2.Name, t2.AnotherId},
                                 (t1, t2) => t1);

Answer 2

llHi you need the following syntax, notice the addition of 'Key'

var Collection = from R in DbContext.TableA  
join G in (DbContext.TableA.Where(r => r.Id == 100)
            .GroupBy(r => new { r.Name, r.AnotherId } )
            .Select(g => new { Name = g.Key.Name , AnotherId = g.Max(o => o.AnotherId) }))  
on new { R.Name, R.AnotherId } equals new { G.Name, G.AnotherId }  
where R.Id == 100  
select R;  

Answer 3

I would recommend using the query syntax for all parts of your query. By doing it this way, your linq query will have a more similar structure to your original sql query. It would look like this:

var query =
  from a in 
    (from x in DbContext.TableA
     where x.ID == 100 
     select x)
  join b in
    (from x in DbContext.TableA
     where x.ID == 100
     group x by x.Name into x
     select new
     {
       Name = x.Key,
       AnotherId = x.Max(o => o.AnotherId),
     })
  on new { a.Name, a.AnotherId } equals new { b.Name, b.AnotherId }
  select a;

Answer 4

saj and Raphael both have found good points:

.GroupBy(r => new { r.Name, r.AnotherId } )
.Select(g => new { Name = g.Name , AnotherId = g.Max(o => o.AnotherId) }))

Groups don't have a Name. Each group has a Key (and the Key has a Name and AnotherId).

Since you want the Max(AnotherId), you don't want to include AnotherId in your grouping Key (same as it is not present in the GroupBy clause of the original query).

.GroupBy(r => r.Name)  //the Name is the Key
.Select(g => new { Name = g.Key, AnotherId = g.Max(o => o.AnotherId) }))