# MySQL STDDEV_SAMP() function

## STDDEV_SAMP() function

MySQL STDDEV_SAMP() function returns the sample standard deviation of an expression ( the square root of VAR_SAMP()). Statistically, sample standard deviation quantifies the amount of variation or dispersion in a dataset for which you have a sample of the whole population.

It returns NULL if no matching rows are found.

This function is useful in -

- It's used when you have a sample of data and want to estimate the variability in the entire population.
- Data Analysis: The primary purpose of the STDDEV_SAMP() function is to accurately calculate the sample standard deviation.
- When comparing different samples or groups, STDDEV_SAMP() is used to assess the significance of differences between the groups.
- Sample standard deviation is used to calculate confidence intervals for population parameters based on sample data.
- STDDEV_SAMP() is used to explore the variation and distribution of data in sample datasets, helping in initial data analysis.
- Sample standard deviation is used in hypothesis testing to test the validity of assumptions and draw conclusions about population characteristics.

**Syntax:**

STDDEV_SAMP(expr);

Where expr is an expression.

**MySQL Version:** 8.0

**Example: MySQL STDDEV_SAMP() function **

The following statement returns the sample standard deviation of 'total_cost' from purchase table.

Sample table: purchase

Code:

```
-- This query calculates the sample standard deviation of the 'total_cost' column in the 'purchase' table.
SELECT STDDEV_SAMP(total_cost)
-- This statement selects the sample standard deviation of the 'total_cost' column.
FROM purchase;
-- This part of the query specifies the table from which data is being retrieved, which is 'purchase'.
```

**Explanation:**

- The purpose of this SQL query is to compute the sample standard deviation of the 'total_cost' values in the 'purchase' table.
- SELECT STDDEV_SAMP(total_cost): This part of the query selects the sample standard deviation of the 'total_cost' column. Sample standard deviation is a statistical measure of the amount of variation or dispersion in a sample.
- FROM purchase: This part specifies the table from which the data is being selected, which is the 'purchase' table.
- The query will return a single value, which is the sample standard deviation of the 'total_cost' values in the 'purchase' table. This value provides insight into the spread or dispersion of the 'total_cost' values in the sample represented by the 'purchase' table. It's important to note that sample standard deviation is used when the data is a subset (sample) of a larger population, whereas population standard deviation considers the entire population.

Output:

mysql> SELECT STDDEV_SAMP(total_cost) -> FROM purchase; +-------------------------+ | STDDEV_SAMP(total_cost) | +-------------------------+ | 345.494814 | +-------------------------+ 1 row in set (0.00 sec)

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STDDEV()

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