SQL Server FOR JSON Path Nested Array - sql-server

Question

We are trying to use FOR JSON Path in SQL Server 2016 for forming a Nested Array from a SQL Query.
SQL Query:
SELECT A,
B.name as [child.name],
B.date as [child.date]
from Table 1 join Table 2 on Table 1.ID=Table 2.ID FOR JSON PATH
Desired Output:
[{
A:"text",
"child:"[
{"name":"value", "date":"value"},
{"name":"value", "date":"value"}
]
}]
However what we are getting is:
[{
A:"text",
"child:" {"name":"value", "date":"value"}
},
{
A:"text",
"child":{"name":"value", "date":"value"}
}]
How can we use FOR JSON PATH to form nested child array.

Answer 1

instead of join use nested query, e.g.:
SELECT A
, child=(
SELECT B.name as [child.name]
, B.date as [child.date]
FROM Table 2
WHERE Table 2.ID = Table 1.ID
FOR JSON PATH
)
from Table 1 FOR JSON PATH
(the query in the question is broken af so this query is just as broken but should give you the idea)

Answer 2

Assuming this schema:
create table parent(id int primary key, name varchar(100));
create table child(id int primary key, name varchar(100), parent_id int references parent(id));
Here is a working solution - abeit more convoluted - that doesn't involve correlated subqueries and only uses FOR JSON PATH:
SELECT
parent.name AS [name],
child.json_agg AS [children]
FROM parent
JOIN (
SELECT
child.parent_code,
JSON_QUERY(CONCAT('[', STRING_AGG(child.json, ','), ']')) AS json_agg
FROM (
SELECT
child.parent_code,
(SELECT
child.name AS [name]
FOR JSON PATH, WITHOUT_ARRAY_WRAPPER
) AS json
FROM child
) AS child
GROUP BY child.parent_code
) AS child
ON child.parent_code = parent.code
FOR JSON PATH
If you have an index on child.parent_id, then using a correlated subquery as suggested, or the equivalent with CROSS/OUTER APPLY might be more efficient:
SELECT
parent.name AS [name],
child.json AS [children]
FROM parent
OUTER APPLY (
SELECT
name AS [name]
FROM child
WHERE child.parent_id = parent.id
FOR JSON PATH
) child(json)
FOR JSON PATH
Both queries will return :
[
{
"name": "foo",
"children": [
{ "name": "bar" },
{ "name": "baz" }
]
}
]