VAR_SAMP

Calculate sample variance. This function computes the statistical variance for a set of values assumed to be a sample of the population.

Syntax

VAR_SAMP(expr)

Description

Returns the sample variance of expr. That is, the denominator is the number of rows minus one.

It is an aggregate function, and so can be used with the GROUP BY clause.

VAR_SAMP() can be used as a window function.

VAR_SAMP() 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, VAR_SAMP(score) 
  OVER (PARTITION BY test) AS variance_results FROM student_test;
+---------+--------+-------+------------------+
| name    | test   | score | variance_results |
+---------+--------+-------+------------------+
| Chun    | SQL    |    75 |         382.9167 |
| Chun    | Tuning |    73 |         873.0000 |
| Esben   | SQL    |    43 |         382.9167 |
| Esben   | Tuning |    31 |         873.0000 |
| Kaolin  | SQL    |    56 |         382.9167 |
| Kaolin  | Tuning |    88 |         873.0000 |
| Tatiana | SQL    |    87 |         382.9167 |
+---------+--------+-------+------------------+

See Also

• VAR_POP (variance)

• STDDEV_POP (population standard deviation)

_{This page is licensed: GPLv2, originally from fill_help_tables.sql}