SQL Server – How to Construct FOR JSON to Convert TOP 1 Record into JSON Object

jsonjson-pathsql server

I want to use a JSON_QUERY with a SELECT TOP 1 query such that the resultant json has that top 1 record in object form, instead of in table form?

For example, the following query (live demo):

CREATE TABLE Trees
(
    [Id] INT,
    [Type] NVARCHAR(100),
    [Height] DECIMAL(2,1)
);


INSERT INTO Trees ([Id], [Type], [Height])
VALUES 
(1, 'Palm', 5.5), 
(2, 'Pine', 6.2), 
(3, 'Apple', 2.5), 
(4, 'Japanese Cedar', 0.5), 
(5, 'Spanish Fir', 0.6);


SELECT
  highestTree = JSON_QUERY(
      (
         SELECT TOP 1
         Id as id,
         Type as type,
         Height as height
         FROM Trees 
         WHERE Height = (SELECT Max(Height) FROM Trees)
         FOR JSON PATH
      )
  ),
  lowestTree  = JSON_QUERY(
      (
         SELECT TOP 1
         Id as id,
         Type as type,
         Height as height
         FROM Trees 
         WHERE Height = (SELECT MIN(Height) FROM Trees)
         FOR JSON PATH
      )
 )
FOR JSON
  PATH, WITHOUT_ARRAY_WRAPPER
;

outputs:

{"highestTree":[{"id":2,"type":"Pine","height":6.2}],"lowestTree":[{"id":4,"type":"Japanese Cedar","height":0.5}]}

But I want:

{"highestTree":{"id":2,"type":"Pine","height":6.2},"lowestTree":{"id":4,"type":"Japanese Cedar","height":0.5}}

Best Answer

Try this one :


SELECT
  highestTree = JSON_QUERY(
      (
         SELECT TOP 1
         Id as id,
         Type as type,
         Height as height
         FROM Trees 
         WHERE Height = (SELECT Max(Height) FROM Trees)
         FOR JSON PATH,WITHOUT_ARRAY_WRAPPER
      )
  ),
  lowestTree = JSON_QUERY(
      (
         SELECT TOP 1
         Id as id,
         Type as type,
         Height as height
         FROM Trees 
         WHERE Height = (SELECT MIN(Height) FROM Trees)
         FOR JSON PATH,WITHOUT_ARRAY_WRAPPER
      )
 )
FOR JSON
  PATH, WITHOUT_ARRAY_WRAPPER
;

Result:

{"highestTree":{"id":2,"type":"Pine","height":6.2},"lowestTree":{"id":4,"type":"Japanese Cedar","height":0.5}}

Postgres 9.4 or newer

Obviously inspired by this post, Postgres 9.4 added the missing function(s):
_{Thanks to Laurence Rowe for the patch and Andrew Dunstan for committing!}

To unnest the JSON array. Then use array_agg() or an ARRAY constructor to build a Postgres array from it. Or string_agg() to build a text string.

Aggregate unnested elements per row in a LATERAL or correlated subquery. Then original order is preserved and we don't need ORDER BY, GROUP BY or even a unique key in the outer query. See:

How to apply ORDER BY and LIMIT in combination with an aggregate function?

Replace 'json' with 'jsonb' for jsonb in all following SQL code.

SELECT t.tbl_id, d.list
FROM   tbl t
CROSS  JOIN LATERAL (
   SELECT string_agg(d.elem::text, ', ') AS list
   FROM   json_array_elements_text(t.data->'tags') AS d(elem)
   ) d;

Short syntax:

SELECT t.tbl_id, d.list
FROM   tbl t, LATERAL (
   SELECT string_agg(value::text, ', ') AS list
   FROM   json_array_elements_text(t.data->'tags')  -- col name default: "value"
   ) d;

What is the difference between LATERAL and a subquery in PostgreSQL?

ARRAY constructor in correlated subquery:

SELECT tbl_id, ARRAY(SELECT json_array_elements_text(t.data->'tags')) AS txt_arr
FROM   tbl t;

How to apply ORDER BY and LIMIT in combination with an aggregate function?

Subtle difference: null elements are preserved in actual arrays. This is not possible in the above queries producing a text string, which cannot contain null values. The true representation is an array.

Function wrapper

For repeated use, to make this even simpler, encapsulate the logic in a function:

CREATE OR REPLACE FUNCTION json_arr2text_arr(_js json)
  RETURNS text[] LANGUAGE sql IMMUTABLE PARALLEL SAFE AS
'SELECT ARRAY(SELECT json_array_elements_text(_js))';

Make it an SQL function, so it can be inlined in bigger queries.
Make it IMMUTABLE (because it is) to avoid repeated evaluation in bigger queries and allow it in index expressions.
Make it PARALLEL SAFE (in Postgres 9.6 or later!) to not stand in the way of parallelism. See:

When to mark functions as PARALLEL RESTRICTED vs PARALLEL SAFE?

Call:

SELECT tbl_id, json_arr2text_arr(data->'tags')
FROM   tbl;

db<>fiddle here

Postgres 9.3 or older

Use the function json_array_elements(). But we get double quoted strings from it.

Alternative query with aggregation in the outer query. CROSS JOIN removes rows with missing or empty arrays. May also be useful for processing elements. We need a unique key to aggregate:

SELECT t.tbl_id, string_agg(d.elem::text, ', ') AS list
FROM   tbl t
CROSS  JOIN LATERAL json_array_elements(t.data->'tags') AS d(elem)
GROUP  BY t.tbl_id;

ARRAY constructor, still with quoted strings:

SELECT tbl_id, ARRAY(SELECT json_array_elements(t.data->'tags')) AS quoted_txt_arr
FROM   tbl t;

Note that null is converted to the text value "null", unlike above. Incorrect, strictly speaking, and potentially ambiguous.

Poor man's unquoting with trim():

SELECT t.tbl_id, string_agg(trim(d.elem::text, '"'), ', ') AS list
FROM   tbl t, json_array_elements(t.data->'tags') d(elem)
GROUP  BY 1;

Retrieve a single row from tbl:

SELECT string_agg(trim(d.elem::text, '"'), ', ') AS list
FROM   tbl t, json_array_elements(t.data->'tags') d(elem)
WHERE  t.tbl_id = 1;

Strings form correlated subquery:

SELECT tbl_id, (SELECT string_agg(trim(value::text, '"'), ', ')
                FROM   json_array_elements(t.data->'tags')) AS list
FROM   tbl t;

ARRAY constructor:

SELECT tbl_id, ARRAY(SELECT trim(value::text, '"')
                     FROM   json_array_elements(t.data->'tags')) AS txt_arr
FROM   tbl t;

Original (outdated) SQL Fiddle.
db<>fiddle here.

Need to select a JSON array element dynamically from a postgresql table

Notes (outdated since pg 9.4)

We would need a json_array_elements_text(json), the twin of json_array_elements(json) to return proper text values from a JSON array. But that seems to be missing from the provided arsenal of JSON functions. Or some other function to extract a text value from a scalar JSON value. I seem to be missing that one, too.
So I improvised with trim(), but that will fail for non-trivial cases ...

PostgreSQL 9.3 – How to Remove Object from JSON Array

To remove all elements from the column images (holding a json array) where 'id' is 'note_1':

pg 9.3

UPDATE items i
SET    images = i2.images
FROM  (
  SELECT id, array_to_json(array_agg(elem)) AS images
  FROM   items i2
       , json_array_elements(i2.images) elem
  WHERE  elem->>'id' <> 'note_1'
  GROUP  BY 1
  ) i2
WHERE  i2.id = i.id
AND    json_array_length(i2.images) < json_array_length(i.images);

SQL Fiddle.

Explain

Unnest the JSON array with json_array_elements() in a subquery using an implicit JOIN LATERAL for the set-returning function. Details:
- PostgreSQL joining using JSONB
- How to turn json array into postgres array?
JOIN to the base table and add another WHERE condition using json_array_length() to exclude unaffected rows - so you don't update each and every row of the table, which would be expensive (and wide-spread) nonsense.

pg 9.4

This gets much easier with jsonb and additional jsonb operators.


UPDATE items i
SET    images = i2.images
FROM  (
  SELECT id, array_to_json(array_agg(elem)) AS images
  FROM   items cand
       , json_array_elements(cand.images) elem
  WHERE  cand.images @> '{[{"id":"note_1"}]}'::jsonb
  AND    elem->>'id' <> 'note_1'
  GROUP  BY 1
  ) i2
WHERE i2.id = i.id;

Eliminates unaffected rows at the start, which is much faster. Plus, there is extensive native index support for jsonb, too, now.

Here are some example, benchmarks and comparison of new features to old json and MongoDB, plus outlook to jsquery by (some of) their main authors, Alexander Korotkov, Oleg Bartunov andTeodor Sigaevat PGCon 2014:

"CREATE INDEX ... USING VODKA"