FOR JSON path returns less number of Rows on AZURE SQL

Question

I am using AZURE SQL (SQL Server 2016) and creating a query to give me output in JSON object. I am adding FOR JSON PATH at the end of query.

When I execute the procedure without adding FOR JSON PATH to the query, I get 244 rows (no of records in my table); but when I execute the procedure by adding FOR JSON PATH I get message 33 rows and also I get JSON object which is truncated.

I tested this with different types of queries including simple query selecting only 10 columns, but I always get less number of rows with FOR JSON PATH and JSON object truncated at the end.

Here is my query

SELECT 
    [Id]
    ,[countryCode]
    ,[CountryName]
    ,[FIPS]
    ,[ISO1]
    ,[ISO2]
    ,[ISONo]
    ,[capital]
    ,[region]
    ,[currency]
    ,[currencyCode]
    ,[population]
    ,[timeZone]
    ,[timeZoneCode]
    ,[ISDCode]
    ,[currencySymbol]
FROM 
    [dbo].[countryDB]

Above query returns 2 rows.

And I use following query to get output in JSON

SELECT 
    [Id]
    ,[countryCode]
    ,[CountryName]
    ,[FIPS]
    ,[ISO1]
    ,[ISO2]
    ,[ISONo]
    ,[capital]
    ,[region]
    ,[currency]
    ,[currencyCode]
    ,[population]
    ,[timeZone]
    ,[timeZoneCode]
    ,[ISDCode]
    ,[currencySymbol]
FROM 
    [dbo].[countryDB] 
FOR JSON PATH

Above query returns 33 rows and output is

[{"Id":1,"countryCode":"AD","CountryName":"Andorra","FIPS":"AN","ISO1":"AD","ISO2":"AND","ISONo":20,"capital":"Andorra la Vella","region":"Europe","currency":"Euro","currencyCode":"EUR","population":67627,"timeZone":2.00,"timeZoneCode":"DST","ISDCode":"+376"},{"Id":2,"countryCode":"AE","CountryName":"United Arab Emirates","FIPS":"AE","ISO1":"AE","ISO2":"ARE","ISONo":784,"capital":"Abu Dhabi","region":"Middle East","currency":"UAE Dirham","currencyCode":"AED","population":2407460,"timeZone":4.00,"timeZoneCode":"STD","ISDCode":"+971"},{"Id":3,"countryCode":"AF","CountryName":"Afghanistan","FIPS":"AF","ISO1":"AF","ISO2":"AFG","ISONo":4,"capital":"Kabul","region":"Asia","currency":"Afghani","currencyCode":"AFA","population":26813057,"timeZone":4.50,"timeZoneCode":"STD","ISDCode":"+93"},{"Id":4,"countryCode":"AG","CountryName":"Antigua and Barbuda","FIPS":"AC","ISO1":"AG","ISO2":"ATG","ISONo":28,"capital":"Saint Johns","region":"Central America and the Caribbean","currency":"East Caribbean Dollar","currencyCode":"205","population":66970,"timeZone":-4.00,"timeZoneCode":"STD","ISDCode":"+1"},{"Id":5,"countryCode":"AI","CountryName":"Anguilla","FIPS":"AV","ISO1":"AI","ISO2":"AIA","ISONo":660,"capital":"The Valley","region":"Central America and the Caribbean","currency":"East Caribbean Dollar","currencyCode":"205","population":12132,"timeZone":-4.00,"timeZoneCode":"STD","ISDCode":"+1"},{"Id":6,"countryCode":"AL","CountryName":"Albania","FIPS":"AL","ISO1":"AL","ISO2":"ALB","ISONo":8,"capital":"Tirana","region":"Europe","currency":"Lek","currencyCode":"ALL","population":3510484,"timeZone":2.00,"timeZoneCode":"DST","ISDCode":"+355"},{"Id":7,"countryCode":"AM","CountryName":"Armenia","FIPS":"AM","ISO1":"AM","ISO2":"ARM","ISONo":51,"capital":"Yerevan","region":"Commonwealth of Independent States","currency":"Armenian Dram","currencyCode":"AMD","population":3336100,"timeZone":5.00,"timeZoneCode":"DST","ISDCode":"+374"},{"Id":8,"countryCode":"AN","CountryName":"Netherlands Antilles","FIPS":"NT","ISO1":"AN","ISO2":

I am trying to get output directly in JSON

Answer 1

When FOR JSON queries are returned to the client, the JSON text is returned as a single-column result set. The JSON is broken into fixed-length strings and sent over multiple rows.

It's really hard to see this properly in SSMS, as SSMS concatenates the results for you in "Results to Grid", and truncates each row in "Results to Text".

Why? Dunno. My guess is that only .NET clients know how to efficiently read large streams from SQL Server, and 99% of the time users will still just buffer the whole object. Breaking the JSON over multiple rows gives clients a simple API to read the data incrementally. And in .NET the fact that the de facto standard JSON library is not in the BCL means that SqlClient can't really have a first-class JSON API.

Anyway, from C#, you can use something like this to read the results:

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Data.SqlClient;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp3
{
    class SqlJSONReader: TextReader
    {
        SqlDataReader rdr;
        string currentLine = "";
        int currentPos = 0;
        public SqlJSONReader(SqlDataReader rdr)
        {
            this.rdr = rdr;
        }
        public override int Peek()
        {
            return GetChar(false);
        }
        public override int Read()
        {
            return GetChar(true);
        }
        public  int GetChar(bool Advance)
        {
            while (currentLine.Length == currentPos)
            {
                if (!rdr.Read())
                {
                    return -1;
                }
                currentLine = rdr.GetString(0);
                currentPos = 0;
            }
            int rv =  (int)currentLine[currentPos];
            if (Advance) currentPos += 1;
            return rv;
        }

        public override void Close()
        {
            rdr.Close();
        }

    }

    class Program
    {

        static void Main(string[] args)
        {
            using (var con = new SqlConnection("server=.;database=master;Integrated Security=true"))
            {
                con.Open();
                var sql = @"
select o.object_id as [obj.Id], replicate('n', 2000) as [obj.foo], c.name as [obj.col.name]
from sys.objects o
join sys.columns c 
  on c.object_id = o.object_id
for json path;
"
;
                var cmd = new SqlCommand(sql, con);
                var sr = new StringBuilder();
                using (var rdr = cmd.ExecuteReader())
                {
                    using (var tr = new SqlJSONReader(rdr))
                    {
                        using (var jr = new Newtonsoft.Json.JsonTextReader(tr))
                        {
                           while (jr.Read())
                            {
                                Console.WriteLine($" {jr.TokenType} : {jr.Value}");
                            }
                        }

                    }

                }
                Console.WriteLine(sr.ToString());
            }



        }
    }
}

Answer 2

With thanks to @David Browne. I found I had to use 'print' instead of 'select'

declare @json varchar(max) = (SELECT * FROM dbo.AppSettings FOR JSON AUTO)

print @json