Generalized way to extract JSON from a relational database? - sql-server

Question

Ok, maybe this is too broad for StackOverflow, but is there a good, generalized way to assemble data in relational tables into hierarchical JSON?
For example, let's say we have a "customers" table and an "orders" table. I want the output to look like this:
{
"customers": [
{
"customerId": 123,
"name": "Bob",
"orders": [
{
"orderId": 456,
"product": "chair",
"price": 100
},
{
"orderId": 789,
"product": "desk",
"price": 200
}
]
},
{
"customerId": 999,
"name": "Fred",
"orders": []
}
]
}
I'd rather not have to write a lot of procedural code to loop through the main table and fetch orders a few at a time and attach them. It'll be painfully slow.
The database I'm using is MS SQL Server, but I'll need to do the same thing with MySQL soon. I'm using Java and JDBC for access. If either of these databases had some magic way of assembling these records server-side it would be ideal.
How do people migrate from relational databases to JSON databases like MongoDB?

Answer 1

Here is a useful set of functions for converting relational data to JSON and XML and from JSON back to tables: https://www.simple-talk.com/sql/t-sql-programming/consuming-json-strings-in-sql-server/

Answer 2

SQL Server 2016 is finally catching up and adding support for JSON.
The JSON support still does not match other products such as PostgreSQL, e.g. no JSON-specific data type is included. However, several useful T-SQL language elements were added that make working with JSON a breeze.
E.g. in the following Transact-SQL code a text variable containing a JSON string is defined:
DECLARE #json NVARCHAR(4000)
SET #json =
N'{
"info":{
"type":1,
"address":{
"town":"Bristol",
"county":"Avon",
"country":"England"
},
"tags":["Sport", "Water polo"]
},
"type":"Basic"
}'
and then, you can extract values and objects from JSON text using the JSON_VALUE and JSON_QUERY functions:
SELECT
JSON_VALUE(#json, '$.type') as type,
JSON_VALUE(#json, '$.info.address.town') as town,
JSON_QUERY(#json, '$.info.tags') as tags
Furhtermore, the OPENJSON function allows to return elements from referenced JSON array:
SELECT value
FROM OPENJSON(#json, '$.info.tags')
Last but not least, there is a FOR JSON clause that can format a SQL result set as JSON text:
SELECT object_id, name
FROM sys.tables
FOR JSON PATH
Some references:
https://learn.microsoft.com/en-us/sql/relational-databases/json/json-data-sql-server
https://learn.microsoft.com/en-us/sql/relational-databases/json/convert-json-data-to-rows-and-columns-with-openjson-sql-server
https://blogs.technet.microsoft.com/dataplatforminsider/2016/01/05/json-in-sql-server-2016-part-1-of-4/
https://www.red-gate.com/simple-talk/sql/learn-sql-server/json-support-in-sql-server-2016/

Answer 3

I think one 'generalized' solution will be as follows:-
Create a 'select' query which will join all the required tables to fetch results in a 2 dimentional array (like CSV / temporary table, etc)
If each row of this join is unique, and the MongoDB schema and the columns have one to one mapping, then its all about importing this CSV/Table using MongoImport command with required parameters.
But a case like above, where a given Customer ID can have an array of 'orders', needs some computation before mongoImport.
You will have to write a program which can 'vertical merge' the orders for a given customer ID.For small set of data, a simple java program will work. But for larger sets, parallel programming using spark can do this job.

Answer 4

SQL Server 2016 now supports reading JSON in much the same way as it has supported XML for many years. Using OPENJSON to query directly and JSON datatype to store.

Answer 5

There is no generalized way because SQL Server doesn’t support JSON as its datatype. You’ll have to create your own “generalized way” for this.
Check out this article. There are good examples there on how to manipulate sql server data to JSON format.
https://www.simple-talk.com/blogs/2013/03/26/sql-server-json-to-table-and-table-to-json/