C＃LINQ使用where子句连接2个表

Question

I am trying to join 2 tables while filtering with where clause. The data looks like this:

Category
Name Selected
A    0
B    1
C    0

SubCategory
Name ParentCategory Selected
s1   A              0
s2   B              1
s3   B              0

Expected results:

ParentCatName SubCatName SubCatSelected
B             s2         1
B             s3         0

Actual results:

ParentCatName SubCatName SubCatSelected
B             s2         1
B             s3         1  <-- should be 0

The code I am using is this:

IEnumerable<Category> result =
    from s in subcategories
    join c in categories
    on s.Parent equals c.Name into t
    from r in t.DefaultIfEmpty()
    where r == null ? false : r.Selected == true
    select new Category
    {
        Name = s.Name,
        Parent = (r == null ? string.Empty : r.Name),
        Selected = r.Selected
    };

EDIT: Something that helped me get clarity was to temporarily rewrite this to see the resulting data structures...

var result =
    from s in subcategories
    join c in categories
    on s.Parent equals c.Name into t
    from r in t.DefaultIfEmpty()
    select new
    {
        s, r
    };

Then I came up with the answer to the filtering of selected categories. See my answer below..

Answer 1

Don't overcomplicate the things. What you are trying to achieve is to filter subcategories by the selected categories. You can get the desired result with the following simple query

var result = from s in subcategories
             join c in categories on s.Parent equals c.Name
             where c.Selected
             select s;

Answer 2

It looks like you're setting it wrong. If r == null then you're setting it to false , otherwise you're setting it to true here: r.Select == true .

Just by reading your query it looks like you may not need that where clause at all.

You probably want something like this:

IEnumerable<Category> result =
    from s in subcategories
    join c in categories
    on s.Parent equals c.Name into t
    from r in t.DefaultIfEmpty()
    select new Category
    {
        Name = s.Name,
        Parent = (r == null ? string.Empty : r.Name),
        Selected = r.Selected
    };

Or if you need to do the null check then do this:

IEnumerable<Category> result =
    from s in subcategories
    join c in categories
    on s.Parent equals c.Name into t
    from r in t.DefaultIfEmpty()
    where r != null //I added the null check here
    select new Category
    {
        Name = s.Name,
        Parent = (r.Name), //I removed the null check here
        Selected = r.Selected
    };

Answer 3

I think, your t value inculudes categoies list. t must be include subcategory list and then you can take selected value of subcategory. So you always get selected value as 1 pls try this:

IEnumerable<SubCategory> result =
    from c in categories
    join s in subcategories
    on c.Name equals s.Parent into t
    from r in t.DefaultIfEmpty()
    where r == null ? false : r.Selected == true
    select new SubCategory
    {
        Name = s.Name,
        Parent = (r == null ? string.Empty : r.Name),
        Selected = r.Selected
    };

OBS:I'm not try this now. But I think works.

Answer 4

Okay. So I did some more looking and came up with this...

IEnumerable<Category> result =
    from s in subcategories
    join c in categories.Where(f => f.Selected)
    on s.Parent equals c.Name into t
    from r in t.DefaultIfEmpty()
    where r == null ? false : true
    select new Category
    {
        Name = s.Name,
        Parent = s.Name,
        Selected = s.Selected,
    };

To filter join to only selected parent category, I have added lambda expression right to that data.