SQL to JSON parent/child relationship - sql

Question

I have a table in my Microsoft SQL Server 2017 that looks like this:
+----+-------+----------+-------+-----------+
| ID | Level | ParentID | IsEnd | SomeText |
+----+-------+----------+-------+-----------+
| 1 | 1 | null | 1 | abc |
| 2 | 1 | null | 1 | asd |
| 3 | 2 | 1 | 1 | weqweq |
| 4 | 2 | 1 | 0 | lkjlkje |
| 5 | 3 | 4 | 1 | noonwqe |
| 6 | 3 | 4 | 0 | wet4t4 |
+----+-------+----------+-------+-----------+
And I would like to output a json string:
[{ ID: 1,
SomeText: 'abc',
Child2: [{
ID: 3,
SomeText: 'weqweq'
}, {
ID: 4,
SomeText: 'lkjlkje',
Child3: [{
ID: 5,
SomeText: 'noonwqe'
}, {
ID: 6,
SomeText: 'wet4t4'
}
]}
]
}]
IsEnd is a flag to know where you reached the last level.

Answer 1

You can use a recursive scalar UDF (User Defined Function) that builds the hierarchy starting from the root.
Here is the stub of an UDF you can start from:
create function dbo.udf_create_json_tree(#currentId int)
returns varchar(max)
begin
declare #json nvarchar(max)
declare #id int, #parentId int, #someText varchar(50)
select #id =[ID], #parentId = ParentID, #someText = SomeText
from dbo.tmp
where [ID] = #currentId
set #json =
(
select [ID], SomeText, json_query(dbo.udf_create_json_tree([ID])) as Child
from dbo.tmp
where ParentID = #currentId
for json auto
);
if(#parentId is null)
set #json = concat(
'[{"ID":' + cast (#id as nvarchar(50)) ,
',"SomeText":"' , #someText ,
'","Child":' , cast(#json as nvarchar(max)) ,
'}]'
)
return #json
end
Populate a table with your input values:
create table tmp ([ID] int, [Level] int, ParentID int, IsEnd bit, SomeText varchar(50))
insert into tmp values
(1, 1, null,1, 'abc' )
,(2, 1, null,1, 'asd' )
,(3, 2, 1 ,1, 'weqweq' )
,(4, 2, 1 ,0, 'lkjlkje')
,(5, 3, 4 ,1, 'noonwqe')
,(6, 3, 4 ,0, 'wet4t4' )
Now you can call the UDF on the first node (with ID=1):
select dbo.udf_create_json_tree(1)
Json result:
Formatted json result:
[{
"ID": 1,
"SomeText": "abc",
"Child": [{
"ID": 3,
"SomeText": "weqweq"
},
{
"ID": 4,
"SomeText": "lkjlkje",
"Child": [{
"ID": 5,
"SomeText": "noonwqe"
},
{
"ID": 6,
"SomeText": "wet4t4"
}]
}]
}]
If you really need to name each child node with the level number (Child2, Childx and so on) you'll probably want to implement a replace logic on "Child" string.