Can SQL Server 2016 extract node names from JSON?

Question

I want to extract a complete list of the node names and their paths from any arbitrary, well-formed JSON document stored as nvarchar value in SQL Server 2016. Is there a reliable way to do this?

For Example, for a json value:

DECLARE @json_doc nvarchar(4000) = '{"Name1":"Value1", "Name2":"Value2"}'

Get this result upon querying @json_doc:

NODE_NAME
$.Name1
$.Name2

Answer 1

Select [key] from default OPENJSON schema.

DECLARE @json_doc nvarchar(4000) = '{"Name1":"Value1", "Name2":"Value2"}';

SELECT [key]
FROM OPENJSON(@json_doc);
GO

| key   |
| :---- |
| Name1 |
| Name2 |

dbfiddle here

Answer 2

To add to this answer.

Get all JSON keys in a column:

SELECT Keys.[key]
FROM dbo.<table_name> t
CROSS APPLY (
    SELECT [key] [key]
    FROM OPENJSON((SELECT t.<column_name>))
) Keys

E.g., Table:

+------------------+
| JSONValues       |
+==================+
| {"myKey":1}      |
+------------------+
| {"myOtherKey":1} |
+------------------+

Query Result:

+------------+
| Result     |
+============+
| myKey      |
+------------+
| myOtherKey |
+------------+

Get all JSON keys where JSON is array of objects:

SELECT DISTINCT Keys.[key]
FROM dbo.<table_name, sysname, sample_table> t
CROSS APPLY (
    SELECT x.[value] [value]
    FROM OPENJSON((SELECT t.<json_colum_name, sysname, sample_column>)) x
) ArrayJSON
CROSS APPLY (
    SELECT [key] [key]
    FROM OPENJSON((SELECT ArrayJSON.[value])) x
) Keys

Table:

+----------------------------+
| JSONValues                 |
+============================+
| [{"myKey":1},{"myKey2":2}] |
+----------------------------+
| [{"myKey3":3}]             |
+----------------------------+

Query Result:

+--------+
| Result |
+========+
| myKey  |
+--------+
| myKey2 |
+--------+
| myKey3 |
+--------+

Answer 3

Using recursive CTE to show everything. Type 4, and 5 (results from OPENJSON function) are arrays and sub_json respectively. Others types are atomic values. Query returns full path, value, key and type of each element in json variable.

WITH
T AS (   SELECT CAST(CONCAT ('$', IIF(TRY_CAST([Key] AS int) IS NOT NULL, CONCAT ('[', [Key], ']'), '.' + [Key])) AS nvarchar(MAX)) AS Path
              , [Key]
              , Value
              , Type
              , 1                                                                                                                   Lvl
         FROM OPENJSON (@json_doc)
         UNION ALL
         SELECT CAST(CONCAT (T.Path, IIF(TRY_CAST(O.[Key] AS int) IS NOT NULL, CONCAT ('[', O.[Key], ']'), '.' + O.[Key])) AS nvarchar(MAX))
              , O.[Key]
              , O.Value
              , O.Type
              , T.Lvl + 1
         FROM T
             CROSS APPLY OPENJSON (T.Value) O
         WHERE T.Type IN ( 4, 5 ))
SELECT Path, T.[Key], T.Value, T.Type, T.Lvl FROM T;

On original question will return:

Path    Key     Value   Type  Lvl
$.Name1 Name1   Value1  1     1
$.Name2 Name2   Value2  1     1

If you use numbers as keys like: SET @json_doc ='{"1":"Value1", "2":"Value2"}'; Results will be wrong:

Path    Key Value   Type    Lvl
$[1]    1   Value1  1   1
$[2]    2   Value2  1   1

Problem here is I use IIF(TRY_CAST([Key] AS int) to identify array index and no named items. Some tricks can be done to prevent it, but I don't wanted overcomplicate this thing. (And using numbers as Key names is a bad idea in my opinion). On a little more complicated example (from http://www.json.org/example.html)

DECLARE @json_doc nvarchar(4000) = '{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}
    ]
  }
}}';

results (omiting value for the sake of clarity):

Path                               Key       Type     Lvl
$.menu                             menu      5        1
$.menu.id                          id        1        2
$.menu.value                       value     1        2
$.menu.popup                       popup     5        2
$.menu.popup.menuitem              menuitem  4        3
$.menu.popup.menuitem[0]           0         5        4
$.menu.popup.menuitem[1]           1         5        4
$.menu.popup.menuitem[2]           2         5        4
$.menu.popup.menuitem[2].value     value     1        5
$.menu.popup.menuitem[2].onclick   onclick   1        5
$.menu.popup.menuitem[1].value     value     1        5
$.menu.popup.menuitem[1].onclick   onclick   1        5
$.menu.popup.menuitem[0].value     value     1        5
$.menu.popup.menuitem[0].onclick   onclick   1        5