MySQL - VARIANCE() Function


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In general, aggregation is a consideration of a collection of objects that are bound together as a single entity.

MySQL provides a set of aggregate functions that perform operations on all the entities of the column of a table considering them as a single unit.

The mean is an average value of given set of number. A variance is defined as the average of Squared differences from mean value.

The MySQL VARIANCE() function is a synonym for VAR_POP() it calculates and returns the population standard variance of all the fields in a particular column i.e. this function treats the rows as the whole population (number of rows as the denominator. If the specified row(s) doesn’t exist this function returns NULL.

Syntax

Following is the syntax of this function –

VARIANCE(expr);

Example 1

Following is an example demonstrating the usage of this function. Assume we have created a table with name MyPlayers in MySQL database using CREATE statement as shown below –

mysql> CREATE TABLE MyPlayers(
	ID INT,
	First_Name VARCHAR(255),
	Last_Name VARCHAR(255),
	Socre_In_Exhibiiton_match INT,
	COUNTRY VARCHAR(100),
	PRIMARY KEY (ID)
);

This table stores the first and last names, country, scores in an exhibition match of a player. Now, we will insert 7 records in MyPlayers table using INSERT statements −

insert into MyPlayers values(1, 'Shikhar', 'Dhawan', 95, 'India');
insert into MyPlayers values(2, 'Jonathan', 'Trott', 50, 'SouthAfrica');
insert into MyPlayers values(3, 'Kumara', 'Sangakkara', 25, 'Sri Lanka');
insert into MyPlayers values(4, 'Virat', 'Kohli', 50, 'India');
insert into MyPlayers values(5, 'Rohit', 'Sharma', 25, 'India');
insert into MyPlayers values(6, 'Ravindra', 'Jadeja', 15, 'India');
insert into MyPlayers values(7, 'James', 'Anderson', 15, 'England');

Following query calculates the population standard variance of scores of all the players in an exhibition match –

mysql> SELECT VARIANCE(Socre_In_Exhibiiton_match) from MyPlayers;
+-------------------------------------+
| VARIANCE(Socre_In_Exhibiiton_match) |
+-------------------------------------+
|                   703.0612244897958 |
+-------------------------------------+
1 row in set (0.02 sec)

Example 2

Following is another example of this function. Assume we have created another table named employee_tbl and inserted records in it as follows –

mysql> CREATE TABLE employee_tbl (
	id INT,
	name VARCHAR(255),
	Work_date INT,
	daily_typing_pages INT
);
mysql> insert into employee_tbl values(1, John, DATE('2007-01-24'), 250);
mysql> insert into employee_tbl values(2, 'Ram', DATE('2007-05-27'), 220);
mysql> insert into employee_tbl values(3, 'Jack', DATE('2007-05-06'), 170);
mysql> insert into employee_tbl values(3, 'Jack', DATE('2007-04-06'), 100);
mysql> insert into employee_tbl values(4, 'Jill', DATE('2007-04-06'), 220);
mysql> insert into employee_tbl values(5, 'Zara', DATE('2007-06-06'), 300);
mysql> insert into employee_tbl values(5, 'Zara', DATE('2007-02-06'), 350);

Now, suppose based on the above table you want to calculate the population standard deviation of the column dialy_typing_pages, then you can do so by using the following command −

mysql> SELECT VARIANCE(daily_typing_pages) FROM employee_tbl;
+------------------------------+
| VARIANCE(daily_typing_pages) |
+------------------------------+
|            5771.428571428572 |
+-----------------------------+
1 row in set (0.00 sec)

Example 3

Let us create a table named student and inserted records into it using CREATE and INSERT statements as shown below –

mysql> CREATE TABLE student (name VARCHAR(15), marks INT, grade CHAR);
mysql> INSERT INTO student VALUES ('Raju', 80, 'A');
mysql> INSERT INTO student VALUES ('Rahman', 60, 'B');
mysql> INSERT INTO student VALUES ('Robert', 45, 'C');

Following query prints the standard population variance value of the marks column of the student table –

mysql> SELECT VARIANCE(marks) from student;
+--------------------+
|    VARIANCE(marks) |
+--------------------+
| 205.55555555555554 |
+--------------------+
1 row in set (0.00 sec)

Example 4

Assume we have created and populated a table with name Sales.

mysql> CREATE TABLE sales(
	ID INT,
	ProductName VARCHAR(255),
	CustomerName VARCHAR(255),
	DispatchDate date,
	DeliveryTime time,
	Price INT,
	Location VARCHAR(255)
);
Query OK, 0 rows affected (2.22 sec)
INSERT INTO SALES values(1, 'Key-Board', 'Raja', DATE('2019-09-01'), TIME('11:00:00'), 7000, 'Hyderabad');
INSERT INTO SALES values(2, 'Earphones', 'Roja', DATE('2019-05-01'), TIME('11:00:00'), 2000, 'Vishakhapatnam');
INSERT INTO SALES values(3, 'Mouse', 'Puja', DATE('2019-03-01'), TIME('10:59:59'), 3000, 'Vijayawada');
INSERT INTO SALES values(4, 'Mobile', 'Vanaja', DATE('2019-03-01'), TIME('10:10:52'), 9000, 'Chennai');
INSERT INTO SALES values(5, 'Headset', 'Jalaja', DATE('2019-04-06'), TIME('11:08:59'), 6000, 'Goa');

Following query prints the population standard variance value of the Price column –

mysql> SELECT VARIANCE(Price) FROM SALES;
+-----------------+
| VARIANCE(Price) |
+-----------------+
|         6640000 |
+-----------------+
1 row in set (0.00 sec)
mysql-aggregate-functions.htm
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