Question

I have a list of menu items in database, like:

ID      ParentID      ItemOrder      Name
-------------------------------------------------------    
 1       null               1        Main_1
 2       null               2        Main_2
 3       null               3        Main_3
 4       null               4        Main_4
 5       null               5        Main_5
 6       1                  4        SubMenu_1.4
 7       1                  5        SubMenu_1.5
 8       1                  8        SubMenu_1.8
 9       2                  1        SubMenu_2.1
10       3                  1        SubMenu_3.1
11       3                  2        SubMenu_3.2
12       3                  4        SubMenu_3.4

Submenu ItemOrder may not necessarily start at 1, it may begin with a 3, and the following menu item order can be 7 and so on.

How to create linq query or lambda expression that will return all parent (zero level) menu items and their first child (if exists)?

Expected result:

ID      ParentID      ItemOrder      Name
-------------------------------------------------    
 1       null             1          Main_1
 2       null             2          Main_2
 3       null             3          Main_3
 4       null             4          Main_4
 5       null             5          Main_5
 6       1                4          SubMenu_1.4
 9       2                1          SubMenu_2.1
10       3                1          SubMenu_3.1

Answer 1

var items = dbContext.Table
    .Where(item => item.ParentID == null)
    .Union(dbContext.Table
        .Where(x => x.ParentID != null)
        .GroupBy(x => x.ParentID)
        .Select(g => g.FirstOrDefault()));