# MySQL STDDEV() function

## STDDEV() function

MySQL STDDEV() function returns the population standard deviation of expression.

The STDDEV() function is used to calculate statistical information for a specified numeric field in a query. It returns NULL if no matching rows 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.
- STDDEV() quantifies the spread or dispersion of values around the mean (average) in a sample.
- When analyzing sample data, STDDEV() helps you assess the distribution of values and identify potential outliers or anomalies within the sample.
- When comparing different samples or groups, STDDEV() is used to assess the significance of differences between the groups.
- In some quality control processes, sample standard deviation helps assess the variability in products or processes.
- Sample standard deviation is used to calculate confidence intervals for population parameters based on sample data.

** Syntax:**

STDDEV(expr);

Where **expr** is an expression.

**MySQL Version:** 8.0

**Example: MySQL STDDEV() function **

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

Sample table: purchase

Code:

```
-- This query calculates the standard deviation of the 'total_cost' column in the 'purchase' table.
SELECT STDDEV(total_cost)
-- This statement selects the 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 standard deviation of the 'total_cost' values in the 'purchase' table.
- SELECT STDDEV(total_cost): This part of the query selects the standard deviation of the 'total_cost' column. Standard deviation is a statistical measure of the amount of variation or dispersion in a set of values.
- 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 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 dataset.

Output:

mysql> SELECT STDDEV(total_cost) -> FROM purchase; +--------------------+ | STDDEV(total_cost) | +--------------------+ | 315.392172 | +--------------------+ 1 row in set (0.00 sec)

**Previous:**
STDDEV_SAMP()

**Next: **
MySQL Aggregate Functions and Grouping - SUM()

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