Question

How would you write this exact SQL query with the new Linq-to-NHibernate Provider (3.x)

SELECT Post.Title, COUNT(DISTINCT Comment.UserId)
FROM Post
INNER JOIN Comment ON Post.Id = Comment.PostId
GROUP BY Post.Title

Here is some SQL if you want to make some tests

DECLARE @Post Table(Id int identity(1,1), Title varchar(200))
DECLARE @Comment Table(Id int identity(1,1), PostId int, Comment varchar(200), UserId int)

DECLARE @PostId int

INSERT INTO @Post(Title)
VALUES ('Test')

SELECT @PostId = SCOPE_IDENTITY()

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment', 1)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 2', 1)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 3', 2)

INSERT INTO @Post(Title)
VALUES ('Test 2')

SELECT @PostId = SCOPE_IDENTITY()

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment', 1)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 2', 2)

INSERT INTO @Comment(PostId, Comment, UserId)
VALUES (@PostId, 'Test Comment 3', 3)


SELECT Post.Title, COUNT(DISTINCT Comment.UserId)
FROM @Post Post
INNER JOIN @Comment Comment ON Post.Id = Comment.PostId
GROUP BY Post.Title

Answer 1

I don’t think it’s currently possible to do the count(distinct x) part.

This is the closest I got:

from comment in session.Query<Comment>()
group comment by comment.Post.Title
      into g
      select new
             {
                 Title = g.Key,
                 Count = g.Select(x => x.UserId).Distinct().Count()
             };

But it produces exactly the same SQL as:

from comment in session.Query<Comment>()
group comment by comment.Post.Title
      into g
      select new
             {
                 Title = g.Key,
                 Count = g.Count()
             };

Which is:

SELECT Post.Title, COUNT(*)
FROM Comment
LEFT JOIN Post ON Post.Id = Comment.PostId
GROUP BY Post.Title

You should post an issue to http://jira.nhforge.org. There’s a lot of work going on with the Linq provider and there’s a good chance to get this construct supported in the near future.