SQL Server For JSON Path dynamic column name - json

Question

We are exploring the JSON feature in SQL Sever and for one of the scenarios we want to come up with a SQL which can return a JSON like below
[
{
"field": {
"uuid": "uuid-field-1"
},
"value": {
"uuid": "uuid-value" //value is an object
}
},
{
"field": {
"uuid": "uuid-field-2"
},
"value": "1". //value is simple integer
}
... more rows
]
The value field can be a simple integer/string or a nested object.
We are able to come up with a table which looks like below:
field.uuid | value.uuid | value|
------------|---------- | -----|
uuid-field-1| value-uuid | null |
uuid-field-2| null | 1 |
... more rows
But as soon as we apply for json path, it fails saying
Property 'value' cannot be generated in JSON output due to a conflict with another column name or alias. Use different names and aliases for each column in SELECT list.
Is it possible to do it somehow generate this? The value will either be in the value.uuid or value not both?
Note: We are open to possibility of if we can convert each row to individual JSON and add all of them in an array.

Answer 1

select
json_query((select v.[field.uuid] as 'uuid' for json path, without_array_wrapper)) as 'field',
value as 'value',
json_query((select v.[value.uuid] as 'uuid' where v.[value.uuid] is not null for json path, without_array_wrapper)) as 'value'
from
(
values
('uuid-field-1', 'value-uuid1', null),
('uuid-field-2', null, 2),
('uuid-field-3', 'value-uuid3', null),
('uuid-field-4', null, 4)
) as v([field.uuid], [value.uuid], value)
for json auto;--, without_array_wrapper;

Answer 2

The reason for this error is that (as is mentioned in the documentation) ... FOR JSON PATH clause uses the column alias or column name to determine the key name in the JSON output. If an alias contains dots, the PATH option creates nested objects. In your case value.uuid and value both generate a key with name value.
I can suggest an approach (probably not the best one), which uses JSON_MODIFY() to generate the expected JSON from an empty JSON array:
Table:
CREATE TABLE Data (
[field.uuid] varchar(100),
[value.uuid] varchar(100),
[value] int
)
INSERT INTO Data
([field.uuid], [value.uuid], [value])
VALUES
('uuid-field-1', 'value-uuid', NULL),
('uuid-field-2', NULL, 1),
('uuid-field-3', NULL, 3),
('uuid-field-4', NULL, 4)
Statement:
DECLARE #json nvarchar(max) = N'[]'
SELECT #json = JSON_MODIFY(
#json,
'append $',
JSON_QUERY(
CASE
WHEN [value.uuid] IS NOT NULL THEN (SELECT d.[field.uuid], [value.uuid] FOR JSON PATH, WITHOUT_ARRAY_WRAPPER)
WHEN [value] IS NOT NULL THEN (SELECT d.[field.uuid], [value] FOR JSON PATH, WITHOUT_ARRAY_WRAPPER)
END
)
)
FROM Data d
SELECT #json
Result:
[
{
"field":{
"uuid":"uuid-field-1"
},
"value":{
"uuid":"value-uuid"
}
},
{
"field":{
"uuid":"uuid-field-2"
},
"value":1
},
{
"field":{
"uuid":"uuid-field-3"
},
"value":3
},
{
"field":{
"uuid":"uuid-field-4"
},
"value":4
}
]