How to get child of master entry from sql table - sql

Question

I have a table for ledgers. It consists of MasterID as unique ID and a parentID for joining.
Model of my table
As in the image my parent ledger is 'A'. 'B' is direct child of 'A'. 'C' is direct child of 'B' and 'D' is direct child of 'C'.
I want a select query to select all childs of 'A'. i.e, result will be B,C,D.
I am beginner in SQL.
I tried some while loop for this, but only direct child was accessible. I am not able make a logic for the requirement.
Thank you in advance.

Answer 1

That'as a typical hierarchical query. Consider the solution that makes use of a recursive cte:
with cte as (
select masterID rootID, masterID, name, parentid
from mytable
where parentID is null
union all
select c.rootID, t.masterID, t.name, t.parentID
from mytable t
inner join cte c on c.masterID = t.parentID
)
select * from cte
As commented by The Impaler, you can change starting condition where parentID is null to the id of another node if needed.
With your sample data, this yields:
rootID | masterID | name | parentid
-----: | -------: | :--- | -------:
1 | 1 | A | null
1 | 2 | B | 1
1 | 3 | C | 2
1 | 4 | D | 3
Note that I kept track of the id of the root object, so it is easier to understand what is going on if there are several roots in your data.
You can also use the root to generate a flat list of children:
with cte as (
select masterID rootID, masterID, name, parentid
from mytable
where parentID is null
union all
select c.rootID, t.masterID, t.name, t.parentID
from mytable t
inner join cte c on c.masterID = t.parentID
)
select rootID, string_agg(masterID, ',') childrenID from cte group by rootID
rootID | childrenID
-----: | :---------
1 | 1,2,3,4
Demo on DB Fiddle

Answer 2

Here is an option that uses HierarchyID
The range keys (R1 / R2) in the final select are optional.
Example
Declare #YourTable table (MasterID int ,name varchar(50), ParentID int);
Insert Into #YourTable values
( 1, 'A', null),
( 2, 'B', 1),
( 3, 'C', 2),
( 4, 'D', 3)
Declare #Top int = 2 --<< Sets top of Hier Try NULL for entire hier
Declare #Nest varchar(25) = '|-----' --<< Optional: Added for readability
;with cteP as (
Select MasterID
,ParentID
,Name
,HierID = convert(hierarchyid,concat('/',MasterID,'/'))
From #YourTable
Where IsNull(#Top,-1) = case when #Top is null then isnull(ParentID ,-1) else MasterID end
Union All
Select MasterID = r.MasterID
,ParentID = r.ParentID
,Name = r.Name
,HierID = convert(hierarchyid,concat(p.HierID.ToString(),r.MasterID,'/'))
From #YourTable r
Join cteP p on r.ParentID = p.MasterID)
Select R1 = Row_Number() over (Order By HierID)
,R2 = (Select count(*) From cteP where HierID.ToString() like A.HierID.ToString()+'%')
,Lvl = HierID.GetLevel()
,MasterID
,ParentID
,Name = Replicate(#Nest,HierID.GetLevel()-1) + Name
,HierID
,HierID_String = HierID.ToString()
From cteP A
Order By A.HierID
Returns