Create JSON from a subquery in snowflake

Question

I want to create a JSON string from a list of value, but I've never worked with JSON before. Please see the image below for my 2 tables, and what I want to create on the right.

I tried this, but it doesn't work (excuse my naivety...but I thought this would be the logical implementation of it)

select a.property_key
      ,to_JSON( select application_ID from tableB where a.property_key = b.property_key) as application_list
  from tableA a

I would appreciate the help. I tried googleing, but I find the snowflake documentation very confusing.

Answer 1

Use the window function array_agg to reduce a column to an array.

create table names (
  last_name varchar,
  first_name varchar
);

insert  into names 
    (last_name, first_name) 
values 
    ('waterman', 'tom'), 
    ('waterman', 'alex'),
    ('waterman', 'david'),
    ('barnett', 'rebecca'),
    ('barnett', 'anne');
    
select
    last_name,
    array_agg(first_name) over (partition by last_name) first_names
from names;

The query above produces the following:

LAST_NAME   FIRST_NAMES
waterman    ["tom", "alex", "david" ]
waterman    ["tom", "alex", "david" ]
waterman    ["tom", "alex", "david" ]
barnett     ["rebecca", "anne"  ]
barnett     ["rebecca", "anne"  ]

You can then reduce that to the unique last_name, first_name pairs using the distinct operator.

select
    distinct 
      last_name,
      array_agg(first_name) over (partition by last_name) first_names
from names;

To convert the array of values returned by array_agg to JSON, you can simply cast the result using ::variant.

select
    distinct 
      last_name,
      array_agg(first_name) over (partition by last_name)::variant first_names
from names;