In a sql query I want to output one column as a json array of child values - json

Question

I have a product table that I query for various fields. There is a ProductTagMapping table that assigns several tags to each product. I would like to add a Tags column to my output row that contains a JSON Array of the associated Tag Names from a ProductTag table. So the result would be:
ProductId: 0
ProductName: Pretty Necklace
Tags: ["gold", "topaz", "fire"]
With a join I would get many identical product rows for each tag:
SELECT p.ProductId, p.ProductName. pt.TagName
FROM Product AS p
INNER JOIN ProductTagMapping as ptm ON ptm.ProductId = p.ProductId
INNER JOIN ProductTag as pt ON pt.TagId = ptm.TagId
I found an almost answer, but it's not correct:
SELECT ROW_NUMBER() OVER (ORDER BY p.SKU) AS Id,
p.Id AS ProductId,
p.Name AS ProductName,
REPLACE(REPLACE((SELECT
pt.Name as TagNames
FROM
ProductTag AS pt
INNER JOIN
dbo.Product_ProductTag_Mapping as ptm ON ptm.Product_Id = p.Id AND ptm.ProductTag_Id = pt.Id
WHERE
ptm.Product_Id = p.Id
ORDER BY
pt.Name
FOR
JSON AUTO
), N'{"pt.Name":', N''), '"}', '"') AS [Tags]
FROM dbo.Product AS p WITH (NOLOCK)
Which yields:
"productid": 8,
"productName": "Gold Tone Earrings",
"tags": "[{\"TagNames\":\"blue\",{\"TagNames\":\"clipon\",{\"TagNames\":\"gold\"]"
How would I approach creating the Tags column correctrly?
Thanks
Abbott

Answer 1

Your code looks alright, but you need to do a JSON_QUERY on your "replace"-string, otherwise it will JSON-ize it as a string which you don't want.
See my version below for what i mean. You also replace wrong word, you probably ment: REPLACE(..., "TagNames", '').
This stuff is if you're using older SQL Server that doesn't support creating arrays or STRING_AGGs. The 2022 has support for creating arrays directly:
create table #products (productid int, productname nvarchar(100))
create table #producttag (productid int, tagid int)
create table #tag (tagid int, tagname nvarchar(100))
insert into #products
values (1, 'Earring')
, (2, 'Blouse')
insert into #tag
values (1, 'Gold')
, (2, 'Metal')
, (3, 'Blue')
, (4, 'Red')
insert into #producttag
VALUES (1, 1)
, (1, 2)
, (1, 3)
, (2, 4)
select productid, productname,
JSON_QUERY(stuff((select ',"' + STRING_ESCAPE(tagname, 'json') + '"'
FROM #producttag pt
INNER JOIN #tag t
ON t.tagid = pt.tagid
WHERE pt.productid = p.productid
for xml path(''), type
).value('.', 'nvarchar(max)'), 1, 1, '[') + ']') tags
from #products p
for json path
I don't like replaces, so instead I create the array string manually by using xml path, and then converts it into a json array.