SQL - STDEV() Function

---

---

The SQL STDEV() function calculates the statistical standard deviation of the fields (numerical values) in a particular column. If the specified row(s) doesn’t exist, then this function returns NULL.

The statistical standard deviation is a measure of the amount of variation or dispersion in a set of data. Mathematically, it is the square root of the average of squared deviations of the items from their mean. Symbolically it is represented by σσ. It is commonly used in statistical analysis to understand how much the data deviates from its mean value.

Syntax

Following is the syntax of SQL STDEV() function −

STDEV(column_name)

Parameters

• column_name − It is the name of the column for which we want to calculate the standard deviation.

Example

Assume we have created a table with name CUSTOMERS as shown below −

create table CUSTOMERS(ID INT NOT NULL, 
NAME VARCHAR(20) NOT NULL, 
AGE INT NOT NULL, 
ADDRESS CHAR(25), 
SALARY DECIMAL(18, 2), 
PRIMARY KEY(ID));

Let us insert r values into it −

insert INTO CUSTOMERS VALUES(1, 'Ramesh', 32, 'Ahmedabad', 2000.00);
insert INTO CUSTOMERS VALUES(2, 'Khilan', 25, 'Delhi', 1500.00);
insert INTO CUSTOMERS VALUES(3, 'kaushik', 23, 'Kota', 2000.00);
insert INTO CUSTOMERS VALUES(4, 'Chaitali', 25, 'Mumbai', 6500.00);
insert INTO CUSTOMERS VALUES(5, 'Hardik', 27, 'Bhopal', 8500.00);
insert INTO CUSTOMERS VALUES(6, 'Komal', 22, 'MP', 4500.00);
insert INTO CUSTOMERS VALUES(7, 'Muffy', 24, 'Indore', 10000.00);

The table will be created as −

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | Kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following query calculates the statistical standard deviation of salary of all the customers −

SELECT STDEV(SALARY) as st_dev
from CUSTOMERS

Output

+------------------+
| st_dev           |
+------------------+
| 3415.65025531987 |
+------------------+

Example

The following query returns all the customers whose salary is greater than two times salary standard deviation −

SELECT NAME, SALARY, AGE,ADDRESS
FROM CUSTOMERS
WHERE SALARY > (SELECT STDEV(SALARY * 2) FROM CUSTOMERS)

Since, we all know from the above example that two times of salary standard deviation is approximately equals to 6831.3004. Hence, while executing the above code we get the following output −

+----------+----------+-----+---------+
| NAME     | SALARY   | AGE | ADDRESS |
+----------+----------+-----+---------+
| Hardik   |  8500.00 |  27 | Bhopal  |
| Muffy    | 10000.00 |  24 | Indore  |
+----------+----------+-----+---------+

Example

Now, suppose based on the above table we want to calculate the standard deviation of distinct (unique) age of the customers, then we can do so simply using the following query −

SELECT STDEV(DISTINCT AGE) as st_dev
from CUSTOMERS

Output

+------------------+
| st_dev           |
+------------------+
| 3.61939221417077 |
+------------------+

Example

In here, we are trying to get the standard deviation of salary of all customers whose age is greater than 24 −

SELECT STDEV(Salary) AS st_dev
FROM CUSTOMERS
WHERE AGE > 24;

Output of the above code is as follows −

+------------------+
| st_dev           |
+------------------+
| 3424.78709800575 |
+------------------+