Postgres 16

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CREATE TABLE tbl (id int, amount int, brand text);
INSERT INTO tbl VALUES
  (1, 10,   'NULL')
, (1, 20,   'NULL')
, (2, 30,   'Mazada')
, (2, NULL, 'BMW')
, (3, 40,   'NULL')
, (3, 40,   'KIA')
, (4, NULL, 'Honda')
, (4, NULL, 'Honda')
;

CREATE TABLE



INSERT 0 8


-- your original
SELECT id, 'Amount' AS difference
FROM   tbl
GROUP  BY id
HAVING count(DISTINCT amount) > 1
   OR (count(amount) != count(*)
      AND count(DISTINCT amount) > 0)
   
UNION ALL
SELECT id, 'Brand' AS difference
FROM   tbl
GROUP  BY id
HAVING count(DISTINCT brand) > 1
   OR (count(brand) != count(*)
AND count(DISTINCT brand) > 0);


id
difference


1
Amount

2
Amount

2
Brand

3
Brand



SELECT 4


-- subtantially faster
SELECT id, col
FROM  (
   SELECT id
        , min(amount) <> max(amount) AS a_diff, bool_or(true) FILTER (WHERE amount IS NULL) AS a_null
        , min(brand)  <> max(brand)  AS b_diff, bool_or(true) FILTER (WHERE brand  IS NULL) AS b_null
   FROM   tbl
   GROUP  BY id
   ) t
JOIN   LATERAL (
   VALUES
     ('Amount', a_diff, a_null)
   , ('Brand' , b_diff, b_null)
   ) x(col, diff, has_null) ON (diff OR NOT diff AND has_null)
ORDER  BY id, col;


id
col


1
Amount

2
Amount

2
Brand

3
Brand



SELECT 4