How to limit a LINQ left outer join to one row

Question

I have a left outer join (below) returning results as expected. I need to limit the results from the \'right\' table to the \'first\' hit. Can I do that somehow? Currently,

Answer 1

This will do the job for you.

from i in db.items
let p = db.photos.Where(p2 => i.id == p2.item_id).FirstOrDefault()
orderby i.date descending
select new
{
  itemName = i.name,
  itemID = i.id,
  id = i.id,
  photoID = p == null ? null : p.PhotoID.ToString();
}

I got this sql when I generated it against my own model (and without the name and second id columns in the projection).

SELECT [t0].[Id] AS [Id], CONVERT(NVarChar,(
    SELECT [t2].[PhotoId]
    FROM (
        SELECT TOP (1) [t1].[PhotoId]
        FROM [dbo].[Photos] AS [t1]
        WHERE [t1].[Item_Id] = ([t0].[Id])
        ) AS [t2]
    )) AS [PhotoId]
FROM [dbo].[Items] AS [t0]
ORDER BY [t0].[Id] DESC

When I asked for the plan, it showed that the subquery is implemented by this join:

<RelOp LogicalOp="Left Outer Join" PhysicalOp="Nested Loops">

Answer 2

What you want to do is group the table. The best way to do this is:

    var query = from i in db.items
                join p in (from p in db.photos
                           group p by p.item_id into gp
                           where gp.Count() > 0
                           select new { item_id = g.Key, Photo = g.First() })
            on i.id equals p.item_id into tempPhoto
            from tp in tempPhoto.DefaultIfEmpty()
            orderby i.date descending 
            select new
            {
                itemName = i.name,
                itemID = i.id,
                id = i.id,
                photoID = tp.Photo.PhotoID.ToString()
            };

---

Edit: This is Amy B speaking. I'm only doing this because Nick asked me to. Nick, please modify or remove this section as you feel is appropriate.

The SQL generated is quite large. The int 0 (to be compared with the count) is passed in via parameter.

SELECT [t0].X AS [id], CONVERT(NVarChar(MAX),(
    SELECT [t6].Y
    FROM (
        SELECT TOP (1) [t5].Y
        FROM [dbo].[Photos] AS [t5]
        WHERE (([t4].Y IS NULL) AND ([t5].Y IS NULL)) OR (([t4].Y IS NOT NULL) AND ([t5].Y IS NOT NULL) AND ([t4].Y = [t5].Y))
        ) AS [t6]
    )) AS [PhotoId]
FROM [dbo].[Items] AS [t0]
CROSS APPLY ((
        SELECT NULL AS [EMPTY]
        ) AS [t1]
    OUTER APPLY (
        SELECT [t3].Y
        FROM (
            SELECT COUNT(*) AS [value], [t2].Y
            FROM [dbo].[Photos] AS [t2]
            GROUP BY [t2].Y
            ) AS [t3]
        WHERE (([t0].X) = [t3].Y) AND ([t3].[value] > @p0)
        ) AS [t4])
ORDER BY [t0].Z DESC

The execution plan reveals three left joins. At least one is trivial and should not be counted (it brings in the zero). There is enough complexity here that I cannot clearly point to any problem for efficiency. It might run great.

Answer 3

Use an inner query. Include DefaultIfEmpty for the case of no photo and orderby for the case of more than one. The following example takes the photo with the greatest id.

var query = 
    from i in db.items
    let p = from p in db.photos where i.id == p.item_id orderby p.id select p).DefaultIfEmpty().Last()
    orderby i.date descending
    select new {
      itemName = i.name,
      itemID = i.id,
      id = i.id,
      photoID = p.PhotoID
    };

If you need to handle the case of no photo specially, you can omit DefaultIfEmpty and use FirstOrDefault/LastOrDefault instead.

Answer 4

You could do something like:

var q = from c in
          (from s in args
           select s).First()
        select c;

Around the last part of the query. Not sure if it will work or what kind of wack SQL it will produce :)