Inconsistent evaluation of NULLIF(ROUND(VAR_SAMP(...)), const) <= value between MySQL and PostgreSQL/DuckDB causes different query results

In MariaDB, the following query returns 5 rows, while the same query returns 0 rows in PostgreSQL and DuckDB:

SELECT COUNT(*)

FROM users AS ref_0

WHERE NULLIF(

        ROUND((SELECT VAR_SAMP(id) FROM posts)),

        86.82

      ) <= 2;

Expected behavior

The query should behave consistently across database engines. Given the semantics of:

• VAR_SAMP(id) (sample variance aggregate over posts)
• ROUND(...)
• NULLIF(..., 86.82)
• comparison with <= 2

The expression inside the WHERE clause should evaluate to either:

• NULL → predicate becomes unknown/false, or
• a numeric value → consistently comparable across engines

In both PostgreSQL and DuckDB, the condition evaluates in a way that results in no rows matching, which is the expected and standard SQL behavior.

Actual behavior (MariaDB)

MariaDB returns 5 rows, indicating that the predicate:

NULLIF(ROUND((SELECT VAR_SAMP(id) FROM posts)), 86.82) <= 2

is evaluating differently, leading to rows being incorrectly included.

• How to Reproduce

  -- schema

  CREATE TABLE users (

  id INT,

  username VARCHAR(100),

  email VARCHAR(255),

  age INT,

  status VARCHAR(20),

  created_at TIMESTAMP NULL,

  score DOUBLE

  );

  CREATE TABLE posts (

  id INT,

  user_id INT,

  title VARCHAR(255),

  content VARCHAR(1000),

  views INT,

  likes INT,

  created_at TIMESTAMP NULL,

  rating DOUBLE

  );

  INSERT INTO users VALUES

  (1, 'alice', 'alice@test.com', 20, 'active', '2022-01-01 10:00:00', 88.5),

  (2, 'bob', 'bob@test.com', 30, 'active', '2022-01-02 11:00:00', 92.3),

  (3, 'carol', NULL, NULL, 'banned','2022-01-03 12:00:00', NULL),

  (4, 'dave', 'dave@test.com', 45, 'active', '2022-01-04 13:00:00', 65.2),

  (5, NULL, 'null@test.com', 18, 'inactive','2022-01-05 14:00:00', 70.0);

  INSERT INTO posts VALUES

  (1, 1, 'Hello World', 'First post', 100, 10, '2022-01-10 10:00:00', 4.5),

  (2, 1, 'Another Post', NULL, 150, 20, '2022-01-11 11:00:00', 3.0),

  (3, 2, 'Bob Post', 'Content', NULL, 5, '2022-01-12 12:00:00', NULL),

  (4, 3, NULL, 'Empty', 50, 2, '2022-01-13 13:00:00', 5.0),

  (5, 4, 'Last Post', 'Last', 300, 30,'2022-01-14 14:00:00', 4.9);

  -- trigger sql

  SELECT COUNT(*)

  FROM users AS ref_0

  WHERE NULLIF(

  ROUND((SELECT VAR_SAMP(id) FROM posts)),

  86.82

  ) <= 2;