Parse JSON text inside NVARCHAR column when using FOR JSON PATH - sql

Question

Suppose that I have a table with a records like this:
CREATE TABLE json_store (
[id] INT IDENTITY(1, 1) PRIMARY KEY,
[json_data] NVARCHAR(MAX)
)
id | json_data
------------------------------------------
1 | {"data_in_json": "Hello from 1"}
2 | {"data_in_json": "Hello from 2"}
3 | {"data_in_json": "Hello from 3", "remarks": "I have a different structure"}
and I want a JSON result like this:
[
{
"id": 1,
"json_data": {"data_in_json": "Hello from 1"}
},
{
"id": 2,
"json_data": {"data_in_json": "Hello from 2"}
},
{
"id": 3,
"json_data": {"data_in_json": "Hello from 3", "remarks": "I have a different structure"}
}
]
I've been using FOR JSON PATH but the problem is it returns JSON as an escaped string.
SELECT [id], [json_data]
FROM [json_store]
FOR JSON PATH
[
{
"id": 1,
"json_data": "{\"data_in_json\": \"Hello from 1\"}"
},
{
"id": 2,
"json_data": "{\"data_in_json\": \"Hello from 2\"}"
},
{
"id": 3,
"json_data": "{\"data_in_json\": \"Hello from 3\", \"remarks\": \"I have a different structure\"}"
}
]
I wanted to know is it possible to parse a JSON column along with the result?
Also, I've tried using OPENJSON() to parse and use FOR JSON PATH convert it back to JSON but it does break the structure into [{"key": ..., "value": ...}, ...]

Answer 1

You need to convert your NVARCHAR(MAX) column json_data to JSON first, using JSON_QUERY, so that SQL Server knows it is already JSON and won't escape it:
CREATE TABLE #T (id INT, json_data NVARCHAR(MAX));
INSERT #T(id, json_data)
VALUES
(1, '{"data_in_json": "Hello from 1"}'),
(2, '{"data_in_json": "Hello from 2"}'),
(3, '{"data_in_json": "Hello from 3", "remarks": "I have a different structure"}');
SELECT id, JSON_QUERY(t.json_data) AS json_data
FROM #T AS t
FOR JSON AUTO;
Which gives:
[
{
"id": 1,
"json_data": {
"data_in_json": "Hello from 1"
}
},
{
"id": 2,
"json_data": {
"data_in_json": "Hello from 2"
}
},
{
"id": 3,
"json_data": {
"data_in_json": "Hello from 3",
"remarks": "I have a different structure"
}
}
]