a window function:

CREATE OR REPLACE TABLE student_test (name CHAR(10), test CHAR(10), score
TINYINT);

INSERT INTO student_test VALUES 
  ('Chun', 'SQL', 75), ('Chun', 'Tuning', 73),
  ('Esben', 'SQL', 43), ('Esben', 'Tuning', 31),
  ('Kaolin', 'SQL', 56), ('Kaolin', 'Tuning', 88),
  ('Tatiana', 'SQL', 87);

SELECT name, test, score, STDDEV_POP(score) 
 OVER (PARTITION BY test) AS stddev_results FROM student_test;
+---------+--------+-------+----------------+
| name    | test   | score | stddev_results |
+---------+--------+-------+----------------+
| Chun    | SQL    |    75 |        16.9466 |
| Chun    | Tuning |    73 |        24.1247 |
| Esben   | SQL    |    43 |        16.9466 |
| Esben   | Tuning |    31 |        24.1247 |
| Kaolin  | SQL    |    56 |        16.9466 |
| Kaolin  | Tuning |    88 |        24.1247 |
| Tatiana | SQL    |    87 |        16.9466 |
+---------+--------+-------+----------------+

URL: https://mariadb.com/kb/en/std/https://mariadb.com/kb/en/std/