SQL Server 2017

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CREATE TABLE Table1
    (first_name varchar(4), last_name varchar(8), city varchar(8))
;
    
INSERT INTO Table1
    (first_name, last_name, city)
VALUES
    ('John', 'Lennon', null),
    ('John', 'Lennon', null),
    ('John', null, null)
;
3 rows affected

select
    case when max(first_name) = min(first_name) then max(first_name) end as first_name,
    case when max(last_name) = min(last_name) then max(last_name) end as last_name,
    case when max(city) = min(city) then max(city) end as city
from Table1
for json path, without_array_wrapper

JSON_F52E2B61-18A1-11d1-B105-00805F49916B

{"first_name":"John","last_name":"Lennon"}

Warning: Null value is eliminated by an aggregate or other SET operation.

select
    case when count(distinct first_name) = 1 then max(first_name) end as first_name,
    case when count(distinct last_name) = 1 then max(last_name) end as last_name,
    case when count(distinct city) = 1 then max(city) end as city
from Table1
for json path, without_array_wrapper

JSON_F52E2B61-18A1-11d1-B105-00805F49916B

{"first_name":"John","last_name":"Lennon"}

Warning: Null value is eliminated by an aggregate or other SET operation.

select
    case when count(distinct first_name) = 1 and count(first_name) = count(*) then max(first_name) end as first_name,
    case when count(distinct last_name) = 1 and count(last_name) = count(*) then max(last_name) end as last_name,
    case when count(distinct city) = 1 and count(city) = count(*) then max(city) end as city
from Table1
for json path, without_array_wrapper

JSON_F52E2B61-18A1-11d1-B105-00805F49916B

{"first_name":"John"}

Warning: Null value is eliminated by an aggregate or other SET operation.

select
    case when count(distinct first_name) = 1 then max(first_name) end as first_name,
    case when count(distinct last_name) = 1 then max(last_name) end as last_name,
    case when count(city) = 0 then 'null' end as city
from Table1
for json path, without_array_wrapper

JSON_F52E2B61-18A1-11d1-B105-00805F49916B

{"first_name":"John","last_name":"Lennon","city":"null"}

Warning: Null value is eliminated by an aggregate or other SET operation.

select
    case when count(distinct isnull(city, 'null')) = 1 then max(city) end as city
from Table1
for json path, without_array_wrapper

JSON_F52E2B61-18A1-11d1-B105-00805F49916B

{}

Warning: Null value is eliminated by an aggregate or other SET operation.