MS SQL Server：将列视为Json

Question

I want a function (MS SQL Server 2016) that can recursively call itself to traverse a tree and to return that traversal as a single Json value. I have a working chunk of code, shown below, but I'd like to do something other than the clunky JSON_MODIFY I've used. Unfortunately, I can't find a way to make it work without it. If you comment out the line of code with the JSON_MODIFY and uncomment the next line, you'll see what I mean.

Is there a better solution?

DROP TABLE dbo.Node;
GO

DROP FUNCTION dbo.NodeList;
GO

CREATE TABLE dbo.Node (
    NodeId INT NOT NULL ,
    ParentNodeId INT NULL ,
    NodeName NVARCHAR(MAX)
);
GO

INSERT  dbo.Node(NodeId, ParentNodeId, NodeName)
VALUES  (1, NULL, 'A'), (2, 1, 'B'), (3, 1, 'C'), (4, 3, 'D'), (5, 3, 'E');
GO

CREATE FUNCTION dbo.NodeList(@ParentNodeId INT) RETURNS NVARCHAR(MAX)
AS BEGIN
    DECLARE @JsonOut NVARCHAR(MAX) = (
        SELECT  n.NodeId ,
                n.NodeName ,
                JSON_MODIFY(dbo.NodeList(n.NodeId), '$.x', '') AS Children
                -- dbo.NodeList(n.NodeId) AS Children
        FROM    dbo.Node n
        WHERE   ISNULL(n.ParentNodeId, -1) = ISNULL(@ParentNodeId, -1)
        FOR JSON AUTO
        ) ;
    RETURN @JsonOut;
END;
GO

PRINT dbo.NodeList(NULL);
GO

The output with the JSON_MODIFY is exactly what I want...

[{"NodeId":1,"NodeName":"A","Children":[{"NodeId":2,"NodeName":"B"},
{"NodeId":3,"NodeName":"C","Children":[{"NodeId":4,"NodeName":"D"},
{"NodeId":5,"NodeName":"E"}]}]}]

... but without it, it all goes wrong ...

[{"NodeId":1,"NodeName":"A","Children":"[{\"NodeId\":2,\"NodeName\":\"B\"},
{\"NodeId\":3,\"NodeName\":\"C\",\"Children\":\"
[{\\\"NodeId\\\":4,\\\"NodeName\\\":\\\"D\\\"},
{\\\"NodeId\\\":5,\\\"NodeName\\\":\\\"E\\\"}]\"}]"}]

Thanks in advance for any ideas.

Answer 1

Two things:

1) The JSON_MODIFY() doesn't actually generate exactly what I want. Using the original code, the JSON_MODIFY adds an element with the name 'x' and the value '' to the value.

2) Although not a perfect answer, it is much cleaner to replace JSON_MODIFY() with JSON_QUERY() (specifically with JSON_QUERY(dbo.NodeList(...), '$'). Not only does it make more sense, this way but it gets rid of the extra 'x' artifact.

SQL obviously know that whatever comes back from JSON_MODIFY isn't really just an NVARCHAR (despite its signature), but I can't find any other way to make SQL think that an NVARCHAR is JSON.