STD(expr)Returns the population standard deviation of expr. This is an extension
to standard SQL. The standard SQL function STDDEV_POP() can
be used instead.
It is an aggregate function, and so can be used with the GROUP BY clause.
From MariaDB 10.2.2, STD() can be used as a window function.
This function returns NULL if there were no matching rows.
As an aggregate function:
CREATE OR REPLACE TABLE stats (category VARCHAR(2), x INT);
INSERT INTO stats VALUES
('a',1),('a',2),('a',3),
('b',11),('b',12),('b',20),('b',30),('b',60);
SELECT category, STDDEV_POP(x), STDDEV_SAMP(x), VAR_POP(x)
FROM stats GROUP BY category;
+----------+---------------+----------------+------------+
| category | STDDEV_POP(x) | STDDEV_SAMP(x) | VAR_POP(x) |
+----------+---------------+----------------+------------+
| a | 0.8165 | 1.0000 | 0.6667 |
| b | 18.0400 | 20.1693 | 325.4400 |
+----------+---------------+----------------+------------+As 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 |
+---------+--------+-------+----------------+- STDDEV_POP (equivalent, standard SQL)
- STDDEV (equivalent, Oracle-compatible non-standard SQL)
- VAR_POP (variance)
- STDDEV_SAMP (sample standard deviation)