The varSamp
function in ClickHouse calculates the sample variance of a set of values. It's commonly used in statistical analysis to measure the variability or dispersion in a dataset, representing how far the values are spread out from their average.
Syntax
varSamp(x)
Official ClickHouse Documentation on varSamp
Example Usage
SELECT varSamp(value) AS sample_variance
FROM (
SELECT 1 AS value
UNION ALL SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 4
UNION ALL SELECT 5
);
This query calculates the sample variance of the values 1, 2, 3, 4, and 5.
Common Issues
- Returns
nan
(Not a Number) if the input contains less than two values. - May produce unexpected results if used with non-numeric data types.
Additional Information
varSamp
is an unbiased estimator of variance for a normal distribution.- For population variance, use
varPop
instead. - Consider using
stddevSamp
if you need the standard deviation instead of variance.
Frequently Asked Questions
Q: What's the difference between varSamp and varPop in ClickHouse?
A: varSamp
calculates the sample variance and uses n-1 in the denominator, while varPop
calculates the population variance and uses n. Use varSamp
when working with a sample of a larger population.
Q: Can varSamp handle NULL values?
A: Yes, varSamp
automatically ignores NULL values in its calculations.
Q: How does varSamp perform with large datasets?
A: varSamp
is optimized for performance in ClickHouse and can handle large datasets efficiently. However, for extremely large datasets, consider using approximate aggregate functions if exact precision is not required.
Q: Is varSamp numerically stable?
A: Yes, ClickHouse implements varSamp
using a numerically stable algorithm to minimize rounding errors, especially for large datasets or values with high variance.
Q: Can I use varSamp with window functions in ClickHouse?
A: Yes, varSamp
can be used as a window function in ClickHouse, allowing you to calculate rolling or sliding window variances over ordered data.