How to get all children of a parent and then their children using recursion in query

Question

I have structure like this:

<Unit>
  <SubUnit1>
           <SubSubUnit1/>
           <SubSubUnit2/>
           ...
           <SubSubUnitN/>
  </SubUnit1/>
  <SubUnit2>
           <SubSubUnit1/>
           <SubSubUnit2/>
           ...
           <SubSubUnitN/>
  </SubUnit2/>
  ...
  <SubUnitN>
           <SubSubUnit1/>
           <SubSubUnit2/>
           ...
           <SubSubUnitN/>
  </SubUnitN/>
</Unit>

This structure has 3 levels: main Unit, SubUnits and SubSubUnits.

I want to select all children by UnitId.
If I search by Unit, I have to get all tree.
If I search by SubUnit1, I have to get SubUnit1 and all children of SubUnit1.
If I search SubSubUnit2, I have to get itself.

Here is my try:

with a(id, parentid, name)
as (
select id, parentId, name
   from customer a
   where parentId is null 
union all
   select a.id, a.parentid, a.Name
   from customer
     inner join a on customer.parentId = customer.id
    )
select parentid, id, name 
from customer pod
where pod.parentid in (
select id
from customer grbs
where grbs.parentid in (
select id
from customer t
where t.parentid = @UnitId
))
union 
select parentid, id, name
from customer grbs
where grbs.parentid in (
select id
from customer t
where t.parentid = @UnitId
)
union
select parentid, id, name
from customer c
where c.Id = @UnitId
order by parentid, id

I use 3 union-words, it is not well but it works. Case structure will have N levels, how I have to get correct result?

Answer 1

DECLARE @Id int = your_UnitId
;WITH cte AS 
 (
  SELECT a.Id, a.parentId, a.name
  FROM customer a
  WHERE Id = @Id
  UNION ALL
  SELECT a.Id, a.parentid, a.Name
  FROM customer a JOIN cte c ON a.parentId = c.id
  )
  SELECT parentId, Id, name
  FROM cte

Demo on SQLFiddle

Answer 2

In case of parent id is a child of itself then we need to use a different query. For example, schema structure is like below

CREATE TABLE customer
(
  id int,
  parentid int,
  name nvarchar(10)
)

INSERT customer
VALUES(1,  1, 'aaa'),
  (2,  1,    'bbb'),
  (3,  2,    'ccc'),
  (4,  2,    'ddd'),
  (5,  1,    'eee'),
  (6,  5,    'fff'),
  (7,  5,    'ggg'),
  (8,  8,    'hhh'),
  (9,  8,    'iii'),
  (10, 8,    'jjj')

In this case, we need to use below query:

DECLARE @Id int = 1 -- your UnitId
;WITH cte AS 
 (
  SELECT a.Id, a.parentId, a.name
  FROM customer a
  WHERE parentid = @Id
  UNION ALL
  SELECT a.Id, a.parentid, a.Name
  FROM customer a JOIN cte c ON a.parentId = c.id
   and c.id != @Id

  )
  SELECT parentId, Id, name
  FROM cte
go