STD

Syntax

STD(expr)

Description

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.

Examples

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 |
+---------+--------+-------+----------------+

See Also

• STDDEV_POP (equivalent, standard SQL)
• STDDEV (equivalent, Oracle-compatible non-standard SQL)
• VAR_POP (variance)
• STDDEV_SAMP (sample standard deviation)