c# - 编写此 linq 查询的更好方法是什么？

Question

SQL：

SELECT node.CategoryId, 
    node.CategoryName, 
    node.Description, 
    node.Lft, node.Rgt, 
    node.ShowOnMenu,
 (COUNT(parent.CategoryName) - 1) AS Level, 
 (CASE WHEN node.Lft = node.Rgt - 1 THEN 'TRUE' ELSE 'FALSE' END) AS Leaf
FROM Article_Category AS node,
Article_Category AS parent
WHERE node.Lft BETWEEN parent.Lft AND parent.Rgt
GROUP BY node.CategoryId,node.CategoryName,node.Description,node.Lft,node.Rgt,node.ShowOnMenu
ORDER BY node.Lft

我的 linq 表达式：

        var list = (from node in DbContext.Categories
                    from parent in DbContext.Categories
                    where node.Lft >= parent.Lft && node.Lft <= parent.Rgt
                    select new
                    {
                        node.CategoryId,
                        node.CategoryName,
                        node.Description,
                        node.Lft,
                        node.Rgt,
                        node.ShowOnMenu,
                        ParentName = parent.CategoryName,
                    } into x
                    group x by new
                    {
                        x.CategoryId,
                        x.CategoryName,
                        x.Description,
                        x.Lft,
                        x.Rgt,
                        x.ShowOnMenu,
                    } into g
                    orderby g.Key.Lft
                    select new
                    {
                        CategoryId = g.Key.CategoryId,
                        CategoryName = g.Key.CategoryName,
                        Description = g.Key.Description,
                        Lft = g.Key.Lft,
                        Rgt = g.Key.Rgt,
                        ShowOnMenu = g.Key.ShowOnMenu,
                        Level = g.Count() - 1,
                        IsLeaf = g.Key.Lft == g.Key.Rgt - 1
                    }).ToList();

我的问题：

1. linq 表达式太长，有两个“选择新”表达式，我想知道如何使它更短？

2. linq 查询对应的扩展方法是什么？我如何用扩展方法表达“来自...来自...哪里...”？

Answer 1

第一个select new .. into x我不明白你为什么需要，尝试删除它并写group node by new...

"from...from"写成这样的 lambda 表达式：

Categories.SelectMany(n => Categories, (n, p) => new { Node = n, Parent = p });