Question

I have a Group table and within that table, there is a ParentId column that denotes a groups parent within the Group table. The purpose is to build a dynamic menu from these groups. I know I can loop and grab the last child and construct a result set, but I’m curious if there’s a more SQL-y way of accomplishing this.

The table has Id, ParentId and Title fields of int, int and varchar.

Basically, a hierarchy may be constructed this way (People is the base group):

People -> Male   -> Boy
                 -> Man
       -> Female

I want to grab the last child(ren) of each branch. So, {Boy, Man, Female} in this case.

As I mentioned, getting that info isn’t a problem. I’m just looking for a better way of getting it without having to write a bunch of unions and loops where I can basically change the base group and traverse the entire hierarchy outward, dynamically. I’m not really a Db guy, so I don’t know if there’s a slick way of doing this or not.

Answer 1

To get the leaf levels for one of many hierarchies, you can use a Recursive Common Table Expressions (CTEs) to enumerate the hierarchy, and then check which members are not the parent of another group to filter to the leaves:

Declare @RootID int = 1

;with cte as (
  select 
    Id,
    ParentId,
    Title
  From
    Groups
  Where
    Id = @RootID
  Union All
  Select
    g.Id, 
    g.ParentId,
    g.Title
  From
    cte c
      Inner Join
    Groups g
      On c.Id = g.ParentID
)
Select
  *
From
  cte g
Where
  Not Exists (
    Select
      'x'
    From
      Groups g2
    Where
      g2.ParentID = g.Id
  );

You can also do this with a left join rather than a not exists

http://sqlfiddle.com/#!6/8f1aa/9