SQL Server - transforming a string into JSON Object in routine - json

Question

I am currently working on a way to get a distribution list for all users registered to our application. We are using a SQL server to store information, however, due to a move to a non-relational DB schema in the near future, most of our data is stored in one field as a JSON string (It's a VARCHAR(max) field that looks like JSON). When we serve this data back to our Java controllers, we convert the String into a JSON Object. As my question would most likely indicate, the list of users is located in this JSON string. While I know I can just do SELECT JSON_DATA FROM MYTABLE to get all entries of this field, convert it in Java, and get the users field that way, I would be essentially returning a TON of data and throwing away 95% of it.
I was wondering if there is a way in SQL Server to parse a JSON string? Essentially what I want to do is with the following table:
<table style="width:100%" border="1">
<tr>
<th>ID</th>
<th>JSON_DATA</th>
</tr>
<tr>
<td>1</td>
<td>{"data":data,"users":["User1", "User2"]}</td>
</tr>
<tr>
<td>2</td>
<td>{"data":data2,"users":["User2", "User3"]}</td>
</tr>
</table>
I want to return from my SQL routine a list of all unique users.

Answer 1

I think this might give you what you need:
Select JSON_QUERY([fieldName], $.users)
Here's a link to check out too: https://learn.microsoft.com/en-us/sql/t-sql/functions/json-query-transact-sql?view=sql-server-2017

Answer 2

Without native JSON support, I'm afraid you're looking a good ol' string parsing. This should get you part of the way: it returns a single string with
"User1", "User2", "User2", "User3"
DECLARE #ThisUserString VARCHAR(255)
, #FullUserString VARCHAR(255) = ''
DECLARE #xmlSTring VARCHAR(MAX) =
'<table style="width:100%" border="1">
<tr>
<th>ID</th>
<th>JSON_DATA</th>
</tr>
<tr>
<td>1</td>
<td>{"data":data,"users":["User1", "User2"]}</td>
</tr>
<tr>
<td>2</td>
<td>{"data":data2,"users":["User2", "User3"]}</td>
</tr>
</table>'
WHILE CHARINDEX('[', #xmlSTring) > 0
BEGIN
-- Find the next set of users, like ["User1", "User2"]
SELECT #ThisUserString =
SUBSTRING(
#xmlSTring
, /*start*/ CHARINDEX('[', #xmlSTring)
, /*length*/ CHARINDEX(']', #xmlSTring) - CHARINDEX('[', #xmlSTring) + 1
)
-- Add them to the list of all users, like "User1", "User2"
SELECT #FullUserString += ', ' +
SUBSTRING(
#xmlSTring
, /*start*/ CHARINDEX('[', #xmlSTring) + 1
, /*length*/ CHARINDEX(']', #xmlSTring) - CHARINDEX('[', #xmlSTring) - 1
)
-- And remove this set from the string so our WHILE loop will end sometime:
SET #xmlSTring = REPLACE(#xmlSTring, #ThisUserString, '')
END
SET #FullUserString = RIGHT(#FullUserString, LEN(#FullUserString) - 2) -- remove the initial comma
SELECT #FullUserString