SQL Generate JSON string using FOR JSON PATH with 2 ROOTs - sql

Question

I have a very simple table containing 5 columns and the table will only hold 1 record at a time. I'm to generate a JSON string from the record and send it to an endpoint.
This is how the JSON string are to be formatted. As you can see it contains 2 'roots' and this is giving me a hard time getting the correct format
{
"fields": [
{
"fieldName": "Brand",
"values": [
"FORD"
]
},
{
"fieldName": "Engine",
"values": [
"V12"
]
},
{
"fieldName": "Location",
"values": [
"Monaco"
]
}
],
"categories": [
{
"fieldName": "Colour",
"values": [
[
{
"name": "Blue"
}
]
]
},
{
"fieldName": "Interior",
"values": [
[
{
"name": "Red"
}
]
]
}
]
}
This is my table containing the 5 columns
I have managed to create 2 separate SQL queries to get the JSON string. But I can't figure out how do it in one select.
SELECT (
SELECT X.* FROM (
SELECT CASE WHEN CarName IS NOT NULL THEN 'Brand' ELSE NULL END AS fieldName,
CarName AS [value]
FROM [dbo].[JSONBODY]
UNION
SELECT CASE WHEN Engine IS NOT NULL THEN 'Engine' ELSE NULL END AS fieldName,
Engine AS [value]
FROM [dbo].[JSONBODY]
UNION
SELECT CASE WHEN [location] IS NOT NULL THEN 'Location' ELSE NULL END AS fieldName,
[Location] AS [value]
FROM [dbo].[JSONBODY] ) X
FOR JSON PATH, ROOT('fields'))
SELECT (
SELECT Y.* FROM (
SELECT CASE WHEN Colour IS NOT NULL THEN 'Colour' ELSE NULL END AS fieldName,
JSON_QUERY('[["' + Colour + '"]]') AS 'value.name'
FROM [dbo].[JSONBODY]
UNION
SELECT CASE WHEN Interior IS NOT NULL THEN 'Interior' ELSE NULL END AS fieldName,
JSON_QUERY('[["' + Interior + '"]]') AS 'value.name'
FROM [dbo].[JSONBODY]) Y
FOR JSON PATH, ROOT('categories'))
And here are the 2 JSON strings:
{"fields":[{"fieldName":"Brand","value":"Ford"},{"fieldName":"Engine","value":"V6"},{"fieldName":"Location","value":"Boston"}]}
{"categories":[{"fieldName":"Colour","value":{"name":[["Blue"]]}},{"fieldName":"Interior","value":{"name":[["Black"]]}}]}
Question 1:
Is it possible to create the JSON string through a single SQL Select? And how can I do it?
Question 2:
If a column value is NULL it is excluded automatically from the JSON string. But I had to add the fieldName to the select and had hoped it would have exclude it from the JSON string if the corresponding field was NULL. However it creates a {}, in the JSON string. And this is not accepted when calling the endpoint. So is there another way to do it when a column value is NULL? I can of course delete it from the JSON string afterwards....
Hope the above makes sense

Answer 1

To do it as a single SELECT you can just UNION ALL the two results together
You can unpivot the values, then check them afterwards for nulls.
Unfortunately, SQL Server does not have JSON_AGG, so you have to bodge it with STRING_AGG and STRING_ESCAPE
SELECT
v.fieldName,
value = JSON_QUERY('[' + STRING_AGG('"' + STRING_ESCAPE(v.value, 'json') + '"', ',') + ']')
FROM [dbo].[JSONBODY] jb
CROSS APPLY (VALUES
('Brand', jb.Brand),
('Engine', jb.Engine),
('Location', jb.Location)
) v(fieldName, value)
GROUP BY
v.fieldName
FOR JSON PATH, ROOT('fields');
UNION ALL
SELECT
v.fieldName,
[value.name] = JSON_QUERY('[[' + STRING_AGG('"' + STRING_ESCAPE(v.value, 'json') + '"', ',') + ']]')
FROM [dbo].[JSONBODY] jb
CROSS APPLY (VALUES
('Colour', jb.Colour),
('Interior', jb.Interior)
) v(fieldName, value)
GROUP BY
v.fieldName
FOR JSON PATH, ROOT('categories');
If you know you will only ever have one row, you can simplify it by removing the GROUP BY
SELECT (
SELECT
v.fieldName,
value = JSON_QUERY('["' + STRING_ESCAPE(v.value, 'json') + '"]')
FROM [dbo].[JSONBODY] jb
CROSS APPLY (VALUES
('Brand', jb.Brand),
('Engine', jb.Engine),
('Location', jb.Location)
) v(fieldName, value)
WHERE v.value IS NOT NULL
FOR JSON PATH, ROOT('fields')
)
UNION ALL
SELECT (
SELECT
v.fieldName,
[value.name] = JSON_QUERY('[["' + STRING_ESCAPE(v.value, 'json') + '"]]')
FROM [dbo].[JSONBODY] jb
CROSS APPLY (VALUES
('Colour', jb.Colour),
('Interior', jb.Interior)
) v(fieldName, value)
WHERE v.value IS NOT NULL
FOR JSON PATH, ROOT('categories')
);
db<>fiddle