Aggregate Functions in SQL

SQL aggregate functions such as SUM, AVG, MAX, MIN, COUNT etc. produce a single value for the entire group of table entries. Aggregate or column functions can also be used along with SELECT command.

SUM

Sum function calculates the arithmetic SUM of all selected values of a given column. For example, to find sum of fee paid by students from STUDENTS TABLE  we would like to use following command 

SELECT SUM(FEE) FROM STUDENTS;

Here fee is a column name in the students table

AVG (AVERAGE)

Using the AVG command we can calculate average of all the selected values of a given column or field. Here one thing is to note that AVG(DISTINCT) eliminates duplicate field values before calculation. 

Using same example here we will find average fee paid by using following statement

SELECT AVG(FEE) FROM STUDENT;

MAX

MAX function calculates the largest of all selected values of  given column. To check the maximum value in fee column we use following command 

SELECT MAX(FEE) FROM STUDENTS; 

MIN

MIN function is used to calculate lowest value in a specified field. Taking same example here we will find minimum fee paid by any student. By this statement

SELECT MIN(FEE) FORM STUDENTS;

COUNT 

This function counts the number of rows present in the output table. The function COUNT with the star(*) counts the number of rows in the resulting table. Whereas COUNT(DISTINCT) eliminates duplicate fields before counting them in the output table. For example to count the total number of students present in the students table we use following statement:

SELECT COUNT(*)FROM STUDENTS;