VARIANCE
Syntax
VARIANCE(expr)
Contents
Description
Returns the population standard variance of expr
. This is an extension to
standard SQL. The standard SQL function VAR_POP() can be used
instead.
Variance is calculated by
- working out the mean for the set
- for each number, subtracting the mean and squaring the result
- calculate the average of the resulting differences
It is an aggregate function, and so can be used with the GROUP BY clause.
VARIANCE() can be used as a window function.
VARIANCE() returns NULL
if there were no matching rows.
Examples
CREATE TABLE v(i tinyint); INSERT INTO v VALUES(101),(99); SELECT VARIANCE(i) FROM v; +-------------+ | VARIANCE(i) | +-------------+ | 1.0000 | +-------------+ INSERT INTO v VALUES(120),(80); SELECT VARIANCE(i) FROM v; +-------------+ | VARIANCE(i) | +-------------+ | 200.5000 | +-------------+
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_POP(score) OVER (PARTITION BY test) AS variance_results FROM student_test; +---------+--------+-------+------------------+ | name | test | score | variance_results | +---------+--------+-------+------------------+ | Chun | SQL | 75 | 287.1875 | | Chun | Tuning | 73 | 582.0000 | | Esben | SQL | 43 | 287.1875 | | Esben | Tuning | 31 | 582.0000 | | Kaolin | SQL | 56 | 287.1875 | | Kaolin | Tuning | 88 | 582.0000 | | Tatiana | SQL | 87 | 287.1875 | +---------+--------+-------+------------------+
See Also
- VAR_POP (equivalent, standard SQL)
- STDDEV_POP (population standard deviation)
- STDDEV_SAMP (sample standard deviation)
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